Cheated on. Feel I'm being Used. Want to get even. Convince me otherwise

In short: A toxic relationship- My ex (a live-in girlfriend) is living with me, despite having broke up with me a month ago. She started dating someone she cheated on me with, emotionally-speaking (they've been texting for almost as long as we were living with each other [ co-worker she met after we started dating] were telling each other they loved each other while we were still together, going on dates, etc) the day after. Literally.

She moved back in with me a few weeks after we broke up(she never moved her things out), but she's still dating him.

While living with him, she complained about him incessantly. He's controlling. She's cheated on the guy with me a lot since they started "dating." In most circumstances, I'm against enabling the cheating thing, but ...I kind of hate the guy. I've been too aware of his presence for months now. And she initiated. And while she keeps trying to tell me that we're just friends, things happen. Partially my fault. She keeps telling me that nothing matter as much as him, not even herself (verbatim).

She keeps telling me that she won't repeat the mistakes that she made with me with the guy that she cheated on me with. We've messed around. A lot. He's "forgiven" her, but we keep on doing it. The more we do, the colder she gets emotionally. Abusive verbally. So a part of me wants to write the guy, telling him about the things that've happened since he's "forgiven" her. About how she initially saw him, to be used to get over her ex (verbatim). About how she's referred to him as her ex, but without the problems multiple times. Partially to mess with him, to mess with the relationship that caused me a lot of anguish in the past half year, but also partially, because I think the guy's toxic for her. Partially because I want her and him to go through a fraction of the stress I went through with her because of the situation. Partially, because retribution. (I was keenly aware of how that relationship was evolving. I hate myself that I didn't stop it, it's driven me mad, but I'm not sure that I could have) At the end of it, I know I only have myself to blame. So. Convince me not to.
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Details

My ex and I were involved for nearly a year. When we met, she was still involved with her ex, her first real long-term bf. She broke it with him eventually and we started dating. Given circumstances, she moved in with me and we were living together for over half a year and things were starting to get better. Then a co-worker of her's started barraging her with texts day and night. He reminded her of her ex-. After a bit, after she told him she was living with me, he still didn't stop and even got to the point where he asked her to move in with him. She almost, then changed her mind. She's ridiculously gorgeous, but is completely run by emotions, she's young (early-mid 20s) and she has ridiculously low self-esteem.

We were good for a while, then it happened again. While we were dating, I believe her when she told me that she never had sex, but they were already in a full-blown emotional relationship. They were already telling each other they loved each other. I know this as a fact. While she was telling me she loved me. I suspected from the beginning something was happening, but..it didn't become serious until a month before we broke up. For a month, she's been picking fights, trying to get me to dump her. She started lying and once when I caught her, I left for a few days because I was livid. We reconciled, then she started getting desperate for him, so I left and she left. I'd expect it to be over by this point. Obviously, I still was burning so I tried to get her back.

Within a few days of being "official" with him and living with him, she came back and ...things happened. Again and again. After a few weeks, she ended up moving back with me. Rather, she moved her body back here. She never took her things. WE have a cat together we foudn when he was days old. She left him, but wants him back. She's still dating him. And things still happened while she was living with me, even though she kept on telling me how she can't make the same mistakes. The ones that she made with me with the guy. Funny thing, we even celebrated out one-year by travelling to a new town.

Idiotically, I still love her. Which is why I'm letting her live with me until she finds her own place. I let her back because when we spoke (begged her, to be honest) because it was a few days before our one year and because when she talked about him, she seemed unhappy, depressed, cynical, and gave up on herself. He uses her for cash even though she's not really financially well off herself. She was trying to apply to graduate school and gave up on it because of the guy. I've since convinced her to apply, found out about all the deadlines, helped her and still am during this process.

Thing is, while I know I can't trust her, I still want her. It's annoying. And her stuff's still here. She's making it seem like I'm an awful person for wanting her not to be here. Pack her things for her then ship it. There're days where she wants to have him come here to help move.

Like I know how I'm trying to portray the situation, but I'm no angel. We've gotten into screaming matches. But I've been faithful to her. Which is the most annoying thing.

Thing is, she's been shielding him from me since they met, when I wanted to meet him and like be friends (ok- neutralize the threat by humanizing me), except he's refused and she's gone along with it. She can't turn him down. With me, she tries to expose me to this fact almost everyday now. Even when the fact was never lost on me. She keeps on trying to tell me that she only thinks of me as friend, but by this point, i feel that it's insulting. And I know that I'm letting her do this to me, but ...she keeps coming back and I let her. She's in a really vulnerable place. She can't afford a new place and I feel that if she moves in with him for real, she'll be stuck in a bad place. I know it seems like one of those ex- things, but I honestly ...fear? for her. Basically, he was her only friend. While dating me, living with me, he knew that because she told him as much, then kept on pursuing. When she tried to make it purely platonic, he denied it, got upset, and she broke. When he was pursuing her after a drunken confession of "love", while she was dating me, he basically harassed her after work until he "convinced" her to leave me, multiple times, stalked her in my apt, etc. When they were dating (I guess are), he cuts into her self-esteem, becomes mad jealous, controls her and tries to change her. And yet, it feels like she's been a better girlfriend to him in the ...well, they were living together for a few weeks, and technically, a month, than in the year she's been with me. It pisses me off. Asks me to give her a ride to work, except when she has a shift with him, because he'll be upset. Has "prohibited" me from going to her workplace (even when we were dating) when he was working. She won't let him feel jealous, even lies to him about how she and I interact while when we were dating, she felt the need to be perfectly and brutally honest. Although to be fair though, she tells me she doesn't and to stop, but when I do, she engages.

I'm at the point where I want to contact him just to mess with him. Rather, mess things up for her. Rather, not "shield" him from the truth of the matter. I know I said I love her, but her being with him...I feel rage. Her spitting it in my face most days, I feel rage. Everytime she talks about him, that she doesn't give half a **** about herself as she does about him (verbatim), I feel rage. Her telling me about how good she wants to be to him or about how she'll make it up to him, etc....I feel rage. I know I should change the door code, pack her things and ship them to her, but ..

I honestly care about her well being and it feels that the thing with the new guy, it'll destroy her. And also, there're moments when it seems good, but after something happens- read- he beckons her- it all goes away and she's closing herself off to me. And while I know that it's the point of no return, probably is, it feels that I've nothing to lose. She's been putting me through emotional hell (slightly hyperbolic, but still) while I don't feel like I deserve any of it, while she's pretty much said the same, she keeps on "exposing" me to the fact of them. Tells me I'm just a friend, then it gets warmer, then things happen, then I'm just a friend, she wants emotions out of it, etc. She keeps telling me that she doesn't want to make the mistakes that broke our relationship with the guy that she broke our relationship for. I'm pissed because it feels now like I'm just a learning experience for her. Someone who's been taking care of her and from whom she's learned, but now that someone like her ex- is back (and she's made cold comparisons to her ex with the guy), I'm disposible. I'm pissed because ... she keeps on seeing the worst in me, ascribing my actions with messed up motives (like I'm stalking her when I ask her to meet me at a particular place, or when we were dating and I tried to surprise her after work, she yelled at me for spying since it was a shift that they both had and I prevented her from hanging with him) or like I'm manipulating her by telling her how I feel- basically telling me I'm an awful person- while she keeps on trying to see the best in him or making excuses for his messed up behavior (oh, he was drunk, don't you understand? He didn't mean to yell. He's just jealous. etc) It's only been slightly over a month since we've broken up. Technically.

I just feel that if this much's happened, I feel I should at least do something to deserve it. At least that way, it makes sense. Maybe I'm just writing this so someone'll convince me out of it. Angry letter. Telling him about the dynamics. Everything she's said about him, about how she was using him to get over her ex, how he's basically her ex- but without the problems/"better", etc.. Telling him that she's messed around with me again. Well, hint it. Although, frankly, I think it'll only cause drama between them, they'll have a fight, he'll "forgive" her, and she'll just be angry at me. I don't even know the point of it besides ...retribution. I've been dealing with this since Sept. Our only real fights have been about this whole situation. But at this point, she's been warring with me in his stead. Right now, I'm just waiting until she has enough to move out then cut ties with her. A part of me still wants to be with her though. I don't know what to do about the cat. She needs him, at least that's how she portrays it, but she's irresponsible and emotional (like to an extreme). [eg. I say my opinion about him somenights, then we get into a fight and she tries to go to his place at like 4am. I convince her not to, but.] And part of me wants her to be happy. But I'll miss the furball like hell, too. But he might just be used as a way to pry herself back into my life. I don't know. I suppose it's a hate/love thing at this point.