Best friend hooked up with with friend-with-benefits

Please help me. I don't know how to feel about this.

This friend was like a sister to me. I trusted her with all of my heart and would never have expected her to do anything like this. It's even worse because not one but two other close friends in the past have done this to me and I really felt like she was different. I took a LOT of measures in making sure she was different because I knew I wouldn't be able to handle that kind of pain again. However,the first friend did this with my boyfriend and the 2nd did it with my ex boyfriend. This is just a FWB... so why does it hurt even more than the other two?

I don't think I had necessarily romantic feelings for him- I had a loving boyfriend at the time.But I did place a lot of value on his attention and friendship. Besides the (AMAZING) sex, he was like a brother to me. But I slowly started distancing myself from him when he kept openly drooling over my best friend/flatmate. I found it very disrespectful and hurtful and when I told her, she even agreed that it was disrespectful. But I still invited him over a lot which led to them talking more and more and I definitely noticed a "switch" in her. I asked her if she was attracted to him and she admitted that she was and that she hated herself for it. I tried to put my jealous feelings aside (I was DYING inside by this point) and encouraged them to go for each other, obviously not meaning it. She told me "I know you wouldn't like it and that's why I can't bring myself to do anything with him. Chicks before dicks." I was really happy with this and with my decision to trust her.

The next week I came back home after spending the weekend at my boyfriend's and caught them passionately in the act. Her room door was open and I saw everything. She knew she was wrong because she immediately jumped up, started crying and begging me to hear her out. I didn't want to hear any of it, I completely blacked out and just went apeshit on her. I don't even remember (or care) what he was doing, my complete focus was on hurting her like she just hurt me. I don't want to think about what I did or said. I moved out immediately and never spoke to her again nor gave her a chance to speak to me. Honestly I hate her but I'm starting to feel guilty because some people say I overreacted.

I absolutely do NOT want to speak to her again much less forgive her (she made me start cutting again when she's the one who got me to stop). But I do want to know that my feelings are justified. I talked to a few people (probably the wrong people) and they basically shamed me saying I overreacted. That really truly hurts. I feel anger, pain, and betrayed but apparently I'm supposed to be completely fine and dandy with what she did just because "he was just a fwb!". I feel like there's a certain amount of pure disrespect involved when a friend sleeps with someone her friend was involved with especially without asking or at least letting her know. And she lied to me saying she couldn't do it. I can't sleep at night and people disregarding my feelings on top of it all makes me severely depressed. Please help.