He said he wouldn't see her again

In advance I'd like to apologize for the length of this content. However, I feel it was important to include everything from the beginning so that maybe it will be easier to understand. I hope you will read to the end and give me your thoughts because I am in desperate need of them. Thank you, truly.
Lucy

In less than a month, I'll have been dating my boyfriend (who will be referred to as Dan) for two years.

Here's the issue:

Before we started dating, Dan was a good friend of mine--so I was all too aware of his day to day life. Before the two of us started dating, he had a relationship with a co-worker (who will be named Ashley). Neither I, nor any of our friends particularly cared for her because the matter of the fact was that while she was playing around with my Dan, she was already in a relationship and HAD been for five years. In other words, this girl was cheating on her boyfriend of five years with Dan. Now at the beginning of this fiasco, Dan was not aware he was the other man. When he found out, he became much more withdrawn from their relationship, however, the problem is that he didn't withdraw completely. Dan continued to see Ashley--for how long, I am unsure. Of course there came a point where it was officially over and they had nothing left to do with each other, but not soon enough, not when it SHOULD have ended.

Ashley moved away and I began to date Dan with full knowledge of his dating past. Now whether this was the right thing to do or not, I can't say. Was I proud of what he had done? No. Did it stop me from loving him? No. I feel maybe I tried to justify his actions by saying that he was not the one who was doing the cheating. I told myself that it was Ashley's fault because she was the temptress--the instigator--and Dan had merely fallen into her trap. Despite my justifications or opinions, I know that what he did was just as bad as Ashley because he did not walk away immediately. Even with all this information, being in a relationship with Dan is what I wanted because he was/is my greatest friend. And I could live with this, I wasn't in fear that he would cheat on me, or endanger our relationship in any way.

Now let me tell you, I've never really been a big believer in the friends with ex's thing, but it's not something I enforce. It's a personal thing for me, and I have no problem if Dan has interactions with his past excursions. I say this because Ashley moved back to town, and she texted him. He told me, and it was fine. Ashley was always the one that started the conversations, but they were scarce; maybe once every two or three months. At some point Ashley began asking Dan to hang out. He told me. I responded saying that was fine. And that was my first mistake. What was going through my head was that I did not want to be stifling, or restrict him from having his freedoms, especially if he believed this girl was his friend. My issue was their history. This was not a normal ex; this was a girl that lied to him and that he began to lie for. When I told him that I was 'fine' with it, he answered saying he most likely wasn't going to hang out with her anyway because he'd rather spend his time with me. Mind you, I was on break from school at my parent's home at the time and he was across the city and worked all day. So seeing each other and hanging out wasn't a daily thing.

As i'm texting him that SAME night that he said he wouldn't go out with Ashley, I somehow uncover that he did in fact go see her. This was not information he volunteered. This is information I pried out of him. He went to go hang out with her, and was messaging me the whole time he was with Ashley, without saying a word of what he was doing. I might have never known if I didn't ask. So I tell him, I tell him that what he did was not okay, and it made me extremely uncomfortable. I told him that I did not like her. He apologized and said he'd never do it again.

Some time passes and Ashley continued to text him, just as scarcely as before. But now every time it's about hanging out with him. He tells me when this happens and asks if it's okay to do so. Each time I have answered that I didn't like the idea but if it's something he REALLY wanted to do, then that was his choice. Each time, he did not go. This last time was about a month or two ago. Ashley texted yet again, wanting to hang out. This time when he asked I became very irritated. Dan knew/knows I do not like this girl. Dan knew/knows that it makes me extremely uncomfortable because of what happened between them. Yet every time he is asked to hang out, he asks me. In my head, Dan is completely aware that I do not like this but he continues to ask because in truth, seeing her, talking to her, and being with her is something he really seems to enjoy much to my disdain. And it's something I am unable to make sense of in my head. This woman, did a terrible thing, to both men. And Dan, at every opportunity wants to go see her. I asked him why this girl was somebody he felt the need to call a friend still. He answered that she was humorous and fun to be around. I became infuriated that he wanted to be associated with her and told him that if he was really dying to see her that bad he didn't need to ask for my permission.
Probably mistake number two.

Thankfully Dan did not go see her, but this was something that stewed in my head and my heart for the next couple of days. Truly I was and am still confounded. I want Dan to be happy, I want him to have his friends separate from me and have a life. But I could not tolerate this woman anymore and the way she was making me feel about myself and our relationship. I confronted Dan and told him that this issue with Ashley was making me very unhappy and that I was not sure continuing to date him was something that I would want to do if this matter pressed on. I told him that over the last couple days I had thought very much about it and breaking up with him had been something that, regretfully, became a viable option. Dan apologized once again and said that this would not occur anymore. He stated that he would not continue to talk to or attempt to see Ashley anymore if it was going to jeopardize his future with me. At least, that's what I surmised from our conversation.

Currently, I am on my last couple days of winter break, and still Dan lives across town and works all day, so once again, seeing each other is not a daily thing. Two days ago, Dan and I were messaging when he says that Ashley has texted him again asking him to hang out Friday night (as of now, it is 1:46 am Saturday morning). Dan said he was going. I never answered Dan after he said that. I exited my messaging app and have not said a word to him since then. Mistake number three.

This means, Dan went to go see Ashley. For all I know, they are still hanging out right now. After he told me he was done with it. I cannot bring myself to action because honestly, I do not know what to do. I have tried to be mature about it, and I hope that I have. I have tried to be reasonable, and talk things out. I have tried to explain. Ashley is someone I do not respect, she is someone I do not like. She is someone who at one time has hurt Dan. And I am angry. I do not know how else to convey what an issue this is for me. And what's even worse is that more than I am angry--I am afraid. I love Dan very much. He's my best friend, but I feel hurt, I feel unimportant, I feel embarrassed and I feel used. My worst fear is that he still bears feelings for this girl. The reason they ended is ultimately because he was rejected by her which means had she not, they might have possibly still been a thing. She used him like a toy and moved away and left him in the dust. And he has held on so tightly to the idea of hanging out with her that he must still care about her. I've always trusted Dan, but when it comes to this girl, I find that I can't, because he has already compromised his morals for her before.

I don't know what to do. I don't know if I even went about this the right way. I can only I assume I did something wrong to make this happen. Did I push too hard? Was I too controlling? Am I making a bigger deal out of this than there is? I'd really appreciate some guidance here. How should I go about this now? As I've said, I was stupid and made the decision not to talk to him for three full days now. I don't know how to talk to him when I think that he could be with her right now.