Moving past cheating

Hello everyone, I want to state first and foremost that I do NOT want to leave my fiance, I just want advice on moving past what they have done in our past. Our relationship started 6+ years ago and things were great at first, but then I started noticing how much time they would spend with this one person on the phone. (The person lived far away from us, several states away.) I figured they were a friend and they were just close. Well, eventually they (my SO) came to me and explained that the person they were talking to was their boyfriend, but things were going south with them and they broke up. This ramped up my depression significantly to the point where I was ugly crying 5+ days a week, working my way through my own emotions about what happened. Fast-forward a year or two and I'm downstairs working while SO is upstairs with a friend (not the person from the phone.) I go up there (I cannot remember why) and the door is locked. I knock and am let in to the room to find out that SO had been doing things of a sexual nature with the friend, I started crying immediately, and SO says that it didn't mean anything. The friend leaves immediately. This escalates my depression again. Fast forward again and we've moved into my grandmothers house and I am working as her provider/caretaker. A smaller event happens where I find out that SO had been planning for an online friend to come over and hookup (I saw them talk about in their DMs.) I confront SO and they say it's just RP. I was sceptical but I left it alone. Fast-forward again to 2019 and SO is spending most of their time talking to someone online, to the point that they were spending new years eve outside on the phone with them untill I came to get them for the countdown. At this point I'm suspicious, suicidally depressed and battling major anxiety. Well, a few months later (late Feb) I had a nervous breakdown, and finally confronted them about everything. They later it all out to me and told me they were in a relationship with the person on the phone, and we're also cheating with my best friend for several years now. (I suspected this as well, but I didn't want to believe my best friend of 10 years had been doing this to me.). The reason SO gave for the excessive cheating are: 1. I (me, the poster) was so depressed that it made them depressed too (or something along those lines, I can't remember their exact words.) 2. They struggle with an attraction to the male body (they are able to suppress it for the most part, but the previous reason had weakend their resolve.) Fast-forward to now and I'm slowly making progress, I'm no longer suicidal, but it's been really slow progress. As for our relationship, were doing good and moving forward together as well, I don't suspect they are cheating currently, but I'm left with deep trust issues regarding any of SOs new friends. Also, were poly now, and together with one other woman who lives across the country, I don't talk to her a bunch but she likes me a lot and she's a good person. I don't know exactly what I need from this, but I guess what I want is support, I feel that I have been wronged excessively and want to be told it wasn't my fault that this happened. TL;DR I want help moving forward after SO has been a cereal cheater. Please don't just come in and tell me to leave them, I know thats the knee-jerk response to hearing about a cheating victim, but I want more than anything to make this work, so PLEASE only good vibes and support. I will also try to answer any questions you may have about the situation for clarity.