Hi everyone, I'm not in a good situation and I'd like a second opinion.

First, I need to make some premises. I'm new to Reddit so don't know how it works; my ex was the one with BPD and I didn't really search for information on my own but relied on the ones she gave me. Also, sorry for my bad English, not my first language.

This will be a little long and I'll thank you for your time if you'll read it all.

So, as I said before, I recently broke up with my ex-girlfriend because she cheated on me with a person close to me (not emotionally) that I'll Gary. For context purposes, I have to start in 2019. Long story short, we all knew each other cause we were classmates. I liked her and told her but got rejected; she had a thing for Gary but she got together with another guy (that was also her worst ex, her words). During that period I tried to remain friends but she ghosted me pretty often so I moved on. And then was Covid so we couldn't see each other at all. Skipping to 2020, she broke up with this guy and, in a moment of fragility, start to hang out and do things with this Gary. He just wanted to have fun and used her until he could. After he got bored he tried to ghost her. She didn't take it well and she tried to take her life multiple times. And he didn't care at all. Just to clarify what kind of person Gary is, once I was with him and she tried while they were on a phone call and he was laughing, telling her it wasn't his business. Some time passed and we started to hang out and, VERY cautiously, got together. Even if it started well it wasn't always easy. During that time she was diagnosed with BPD and my problems (I have tons, beyond an anxiety disorder that I can control quite well) weren't a priority anymore. Still, I tried my best to support her. Unfortunately  (not that I could avoid it) Gary was and still is in my circle of people so even after all that happened they got along and stayed friends, but he travels quite a lot so he wasn't always around. Well, you all know where this is going. It was obvious that she still had a thing for him. I saw it coming and started to make complexes. I even gave her the possibility to break up and just peacefully be friends because if it ended otherwise I thought I couldn't stand it. All of this without precluding them from the possibility to hang out without me. I hoped it was just me and I could trust them. She always told me that he was special to her and, in the last months of the relationship, that she loved him only platonically. 

There was this party to which we all got invited. He just came back from one of his trips and we didn't see him for some months. She got drunk and when he came they just hung out leaving me on my own. The party ended and around 4 am I brought her to her home. The morning after she called me crying telling me that they saw each other after and they cheated on me. We spoke about it and I found out that he was her FP (before then I didn't know the existence of it and what it means) and they (as she said, him especially) were used to flirt during our relationship.

Let's put aside the frustration of giving your whole in an already difficult relationship and still ending like this. This is the reason I wrote this in the first place. I'm really sorry to anyone who is in this condition, and I don't want to sound like I lack empathy, but I can't accept it's all disorder's faults. I mean, I could understand if it was a moment of impulsivity but it was months of hiding things and lying. So my questions are: where is the limit to give BPD the fault? And when is a person's fault? Also, she admitted that I was the first her first non-toxic relationship and, from my pov, this just complicates things. We agreed that her ex and Gary were pretty toxic, but why would our bond be weaker than a toxic one? Especially when, thanks to therapy, she already knew they were toxic and dangerous for her. How didn't she have in entire months a moment of rationality and be aware of what she was doing?

Anyway, a couple of months have passed since we broke up. She attempted suicide again but luckily failed. Now she is a little better and moved on a bit. We still hang out and I don't know how to behave. I don't know if I have the courage to get back with her and I'm scared that if we'll stay just friends I'll make her suffer more. I feel like if I make any mistake it can cost her life and as you may imagine it's not the best feeling in the world.

I hope I didn't offend anyone. Thanks in advance to anyone who read until here and the ones who shared their opinions. 

Have a nice day.