Hello everyone,
I have been following this forum for a while but this is actually the first time I post here because I have no idea how to handle this situation. I would greatly appreciate your help. Please excuse my english I am not a native speaker!
Let's call me Paul
I started a Master in August and decided to share a flat with this girl I met right before uni, about 7 months ago. As I had already lived with a girl in the past and it went extremely well, I thought that living with her would be the same and could also open opportunities to meet her girl friends she would invite

. At that time she was "kind of dating" a couple of guys with no strings attached. Unfortunately she happens to be really hot (like one of the hottest chicks I have seen), and I happen(ed) to be pretty good at gaming. What happened next is that within the first couple of weeks I started feeling more and more attracted, until we hooked up at home after a party and an oily back and butt massage on the couch.
Since then, she ditched one after the other the two other guys she was seeing (one even flew from Germany to visit her and she told him she "didn't feel it anymore". We slept together literally every day and every night for the next 6 months. We would always play that little game of saying "hey, you're free to do what you want, but just be respectful and don't bring anyone home". We never brought anyone home, we never slept out. We were basically a couple without ever defining the relationship.
About 2 months into that "SPAM-sex-friendship", we stopped hiding from our uni friends, made out several times in public, started holding hands, kissing at uni, and do all the shit couples do. She started hinting me stuff like "I wonder what language our kids would speak" (we both speak 4-5 different languages), "yes that's exactly what moms and girlfriends do", hints that I never catched because even though I was madly in love with the girl, I have been so hurt in a past relationship that
I stupidly decided I wouldn't make the move to officialize this to keep some pressure off it. However, I never felt the need or want to date or sleep with other girls, these were truly some of the 5 happiest months of my life.
After 5 months that seemed perfect on my side, we are both separated for more than a months with our respective classes for study-tours ) and company visits (her in Asia, me in South America.
This is where shit started going down.
We were separated but texted everyday and occasionally SPAM during that month. When we came back, I didn't see it at first but she started acting very cold. One night, a week after we are both back in our home, she comes back wasted from a party (I was at another party) and lies down in bed all dressed up asking:
"Babe, I want to know now, what are we?".
At that moment I don't know if I wanted to avoid the bullet to like "keep the control" or if I just wanted to have this convo in a sober state, I just answered "I don't want to talk about this now". We didn't bring it up the next day.
What happens next is that I find out some days later that that exact night she asked me the question, she made out with a guy from uni she met during the study tour. Apparently he's been teasing and trying (he's a PUA) and she was resisting him all the time during the study tour. However the fact that I never defined the relationship despite her hints even when we were back living together + the fact that he kept on saying "hey, he's just your SPAM, that's what he says right, c'mon

" made her fall for him.
When I heard that she kissed him I freaked out and confronted her badly. She apologized and we tried to make it work after that. However, I got drunk with her one night at a party and I wanted to "officialize" our relationship. Here I made the stupidest move ever: I told her I also made out with a girl on my study tour in South America and I wanted to start this relationship with a "clean slate" and be totally open because I made a mistake too.
She got extremely mad, saying that I made her feel like shit for weeks when I wasn't an angel either, that we were so in love at that time and I kissed another girl, that she kissed when shit was already going down blablabla.... Here starts a war of who is more to blame.
Again, I try to rationalize the thing and say "ok, we both fucked up, now I want to be with you, let's start this fucking clean". I didn't realize I lost her already long ago, she said she was ok to try but to take it slow. However, I already saw she wasn't the same girl anymore. At that point, I became super clingy and needy and worse than all, I didn't trust her anymore. This is how I started checking her phone (never did that in my life but I felt a horrible urge to do so) and saw she was still talking with the guy and flirting hardcore. She was also planning on seeing him again behind my back for dates, and writing her friends "yeah I really like the new guy but I don't want to hurt Paul".
When I find out the messages I decide to just leave the flat for a few days without too much explanation given. I also start being very talkative about the problems in our "relationship" if we can call it such, and telling people, including those that are more friends with her than with me, that I consider moving out. When she finds out that I consider moving out she totally freaks out, breaks it off with me and says "we are never gonna repair this I am so done". She also told me she was in love with me before and wanted this relationship and that now it's all changed, that I didn't make a move on time, that she now feels she wants to enjoy her last months of studying in this city, that she feels like "in a cage" in this flat with me etc...
I freaked out, cried, ran away, begged. Starts a period of 2 weeks bargaining with her, saying that I miss having her as my girlfriend (to which she harshly answered "I was never your girlfriend"). She started going out till very late every night, dresses nice and stuff. We live in the same flat and at that time we slept in different rooms for 3 consecutive weeks. One day, I make another big mistake of stalking her phone again. She had been on several dates with the study tour guy, he was playing her PUA style, she was planning on sleeping at his place soon. And so she did one night. Now they are basically dating, I left the flat to go at a friend's place as I thought I was going to die if I stayed one more second in that house and I'm now looking for a new spot. She is so uninterested now that she even said stuff like "you don't really have to leave the flat for me, you know? We can be friends.". But she does not really mean it, our conversations in the last weeks have been extremely cold and awkward, it's like we are not the same people anymore.
Ever since she slept with him I have been extremely depressed, and so overwhelmed with my feelings that I've been telling everyone at school how bad I felt. She gave me shit for that too because it made her "look bad" and she thinks I badmouthed her, when I really just said that I was devastated and didn't understand how she could go from "I wonder what our kids would speak at home" to total indifference towards me. On her side, I hear from sure sources that never ever shares a thought about me with her friends, at least not in a nostalgic way. She literally doesn't seem to give a shit, a drastical change compared to like a month ago.
I now traveled abroad to see my family and try to reconstruct myself because I felt like a sleepless zombie for the last couple of weeks. However, I feel like it's gonna be really hard to stop being an emotional mess when I come back considering that I see her and the new guy every single day at school. Furthermore, I invested so much in this girl that she also became my best friend in town and most other friendships I have are superficial and are often "friends of our couple". She is no way as affected as I am by this breakup and I am scared that I am going to isolate myself from my social circles now that I lost her my "girlfriend", my best friend, my flat, my dignity, and that I'm annoying everyone with my grieving.
I apologized a lot in the hope of getting her back but I also realized that she behaved like a bitch. However, we had a connection that I haven't felt with anyone else and I don't know what my next move should be. I am trying to go NC but we have to do some paperwork for the flat, money, and I get to see her and him everyday during the week and almost every weekend as we go to the same parties (my uni is very "united" let's say).
Sorry for the long post and possible lack of clarity but it's also still very messy in my head and this story had a lot of subtleties, miscommunication problems and nonesenses (like her always chasing me to be my gf and now her rejecting me harder than stone, wtf.).
Ps Additional info: it's also her birthday in 3 weeks.