First, I know I am a horrible person, so please please don‘t tell me this again, because I already feel horrible. I just wanted to ask if someone can understand why she chooses to forgive him everything so easily and only blames me, altough it needs two to cheat? Maybe I just don‘t understand what she‘s going trough or idk? But it drives me crazy and I can‘t handle my everyday life since then and my BPD makes the situation so much worse. 

Long story short: Some weeks ago I wrote her a long letter to tell her what happend a few months ago and also explained what I was going trough and that he had talked me into having sex with him. I was going trough some rough months with depression, more suicidal thoughts than ever and simultaneously a hard addiction to alcohol. Once we were on a outdoor party together and I drank a lot. Combined with my anitdepressants I had the worst breakdown of my life and tried to kill myself in the nearby lake. He pulled me out before I could seriously harm myself and tried to talk me trough. Eventually I got better and he asked me if I wanted to go swimming naked. I was still more than drunk and didn’t think of it a lot and agreed. Then one thing led to another and we had sex. Since that day I somehow felt a emotional bond between us, above all because he was there for me on that day. He texted me everyday telling me I am so special, he can‘t forget me, he can‘t stop thinking about me, but after all his compliments he also always tried to go in a sexual direction again. He didn’t leave a day out to tell me how special I am and I really felt more than special. I met him again 2 times, mostly because I was trapped in the feeling of being admired so much, but then I decided that I can‘t do that to my sister again and blocked him. I could not live with the shame and also told my sister that I can’t have any contact with her anymore, because I somehow knew she would only blame me. I went to therapy, quitted drinking and started to recover. Nevertheless he always found a way to contact me trough various ways and new accounts, but I did‘t respond. One day I decided I can‘t stand it anymore and I have to tell her the truth and that he is still contacting me. 

Altough I thought I don‘t deserve a second chance I wanted to tell her the truth, so at least she can decide what she wants to do with her future. Her boyfriend never tought of telling her the truth, but she just blames me for not telling her and telling her to late. 

I was extremely scared that she will abandon me; however, I can‘t help myself but I am so disasspointed that she has forgiven her boyfriend just in a few days and decided that it is only my fault not his. I am feeling so lost and hurt (altough I know I have hurt her much more in the first place). 
Please stay kind and tell me your perspective, thanks!