girlfriend was raped by my assaulter when we were kids and I feel guilty

TW for mentions of CSA, abuse, self harm, rape

context: I have BPD and it affects my reaction to the abuse. think of the most intense reaction to finding out the person you love the most was abused by your best friend in the whole wide world (who also assaulted you but whatever) and times it by ten.

ive known my gf and our abuser since 2018/19. the abuser was incredibly sexual from the get go and was a few months older than us (we were 12/13, he was 13/14). we started dating in April of 2022, and then my family moved away for two years. in 2023, I learned the full extent of my girlfriend's abuse. the guy who assaulted me raped her for *multiple years* before switching over to just assaulting and sexually abusing her for even longer.

I just feel so guilty, because the abuser tried to get me involved multiple times. im kinda dense, so I never really picked up on it until way later. he told me a little bit about the abuse all the way back in 7th grade. (all three of us are trans, me and the abuser are transmasc and my girlfriend is transfem) he asked me "can you get pregnant from pre-cum?" and i was so disgusted i slapped him. he simply looked at me and said "i liked that". its just so so gross.

so, when I learned about what he did to her, I cut him off completely. it took forever to get her to cut him off. I became defensive, her guard dog. I snapped at anyone and everyone who even slightly doubted her. I craved justice - but justice never came. lost many "friends"​ because I couldn't get over what happened to her. fuck them, anyways. im just so guilty because I could've stopped it. he practically laid it out to me that he was abusing her back in 7th grade, and instead of telling an adult I held myself back and then forgot. I fucking forgot. I can never forgive myself for that. she could've made it through school healthier than she did if I said something. I feel guilty for "stealing" her grief. she says she forgives him and wants him to get better (he was extremely mentally ill and suicidal). but I cant. I just cant. I hate him and his friends and I hate myself. I used to cry everyday about it and now I only cry once a week about it. its been 3 years since I found out. Im only now in therapy. I hurt myself as a punishment and to release my intense feelings. I feel awful for even doing this, he only coerced me once into acting sexually towards my girlfriend before I was ready - he never raped me. why must i be the one who cries everyday and seeks assurance everyday and be the one with intense PTSD triggers that pop up everyday????

no matter what i do, everything around me will forever be a constant reminder of my failure to protect the only person who loves me. I will bring this shame to my grave. I dont know what to do anymore.