Hi!

Sorry if this is the wron thread to post my problem, but i do belive my mental helth is being compromised because of this issue.

I'm 26 years old and i am living with my girlfriend for more than 1.5 years now. She is the best person i ever knew but we almost broke up yesterday, again. Here's why:
I lived with my family, 2 brothers, 1 sister for all of my life. My parents are very caring and i definately lived in a happy home.
My father is a hunter and his protege was now my girlfriend (28) who is just finishing biology college this year. My mother suspected she had something with my father (she never did) and didn't like her at all. The same goes for me, i actually thought she was very annoying and very "pushy" - she's very talkative and i am more of a quiet person.
There are quite alot of family parties at our house (for our's and my aunt's and uncle's families birthdays,...) and she drank alot those days.
When she was drunk, she became grabbing and hugging and she claims she doesn't remember this a few times reoccuring thing. I was watching F1 race on my bed one day and when she came all drunk into my room (we didn't get along, i thought she was kinda whorish) and started grabbing and hugging me. And when i got her off me a few times she just did it again until finally she fell asleep. It felt really weird but a bit good at the same time.
One day we were in our basement smoking some pot and we really fell for eachother. And i had a really huuuge crush for her in the next two weeks. But then one night my brother told me he slept with her two times and hinted that i shouldnt be with her. They were both drunk. At that moment i felt so sick, and when she came to my room to see me, i asked her why she didn't tell me, she told me she thought i already knew. In fact my whole family knew except me. I told her to leave and i felt gutted the whole night. But i was still very much in love with her (she is actually my first) and we quickly picked up our relationship (constant messaging through SMS and gmail chat).
My family didn't approve and my mother called her a ***** in front of me one time and we decided to leave. We moved to a small rented appartment in a distant village. I got an email from my mother saying what a ***** she is, that everyone from my home village knows this, first father had her (didn't) then my brother, now you and so on.... really nasty stuff i never expected from her. My other brother also sent me an email saying, she was also grabbing him and my cousing, even my aunt at some point.
But i was still very much in love with her and i'm not quite sure how i got through all of this now, when i think about it. But we hava a great realationship otherwise, we have the same goals, values, interests. She stopped drinking after that and quaranteed me she never will get so drunk ever again. She told she had serious trouble getting over her boyfriend, with whom she lived for more than 2 years.
Over the course of our living together, i got really depressed, sad, annoyed, sick by what has happened a lot of times now.
I asked her about her past relationships (she had less than 20) and half of those her boyfriends cheated on her with her best friends, so she never was in a really happy realtionship. When she told me about her family, i was really stunned at what she's been through. Her father, who is cheating on her mother since before she was born, was never around, told her she can't do anything, and did nothing to protect her from her older brother who was beating her up over nothing. So she left home when she was 16, did drugs and quit high school. She has been through alot. I had a much, much, much easier life that she did, everything was kinda always there for me.
Anyway, we got closer with our family again through time, my mother and her get along now, she admitted she was wrong about her and that she knows she is a good person.
But it's so hard for me to see my brother and her at the same place and often when we leave from there i feel gutted again. At one point i asked her what they did, she told me they had a meaningless drunk sex (without a condom, ended up with a handjob...), which she barelly remembers and that she never did that before. Sick image...
My problems continue. I can't let go of her sleeping with my brother, when i think about it i feel so sick in my head, that i don't know if i'll ever be able to get through this. And it's not fair to her, i am 100% positive she loves me very much, she would never cheated on me, she says she has never been happier and i belive her. We have a good relationship except for those times when i break up and start crying uncontrollably. I hide my thoughts from her alot of the time now as i don't want to put her through this again. She then starts blaiming herself and always says she screwed up the best thing that ever happened to her before it even began.
I don't think i'll ever be as happy as i could be because of this and my attempts of getting over it failed. I wonder what it would be like if this never happened. I have too many moments torturing myself with this. We almost broke up again yesterday (3rd time now) and i don't know what to do anymore. I want to be happy and i want her to be happy, but i don't know if this is something i'll ever be able to get over.

EDIT: Might be important for some reason. We didn't have sex until after we lived together (after about 1 month of living together), we both wanted to take it slow.

Please help me/us out with any suggestions. I really appreciate it!

Thank You!