my ex with BPD were involved for two years and dated for about 10 months. everything was going very well and they were very affectionate, attentive, and we got into very few arguments. our friendship and relationship was everything i wanted and i felt loved in return. a week before he is supposed to move away for a job he tells me he cheated on me with a friend of his. the person he cheated with knew him and i and he was blacked out (he had a problem with alcoholism at the time) and off his medication. he has cheated previously on past partners but those relationships were more undefined and he has used that as an excuse, while ours was not and we were very officially together. i was angry and broke things off immediately. weeks later, he attempted suicide and when i called him he said he felt taken advantage of and assaulted by the girl he cheated with. he also told me he did not have sex with her and he began therapy/stopped drinking so i gave him a chance to reconcile. after a few months something in me felt off and i asked him whether he was downplaying the situation. he revealed that he did in fact have sex with her. i reached out to the girl who previously ignored my messages and demanded to know if she took advantage. she let me know he initiated the entire thing and texted her inviting her to go back to his place. i know they were both in the wrong but i am so confused and hurt by the situation. my ex w BPD  made such an effort with me and i felt very loved and had no reason to doubt our relationship. i know he feels remorseful and awful. his only answer when i ask him why is "im sick in the head." i don't want him back but i don't understand how things went so wrong and i don't understand the lies that were used to cover it up. just looking for some insight and using this as an outlet to vent.