I'm in a messed up situation.. Any advice?

To try to make a long story short me and my girlfriend bought a house together. We have been going out for 5 years. But once we got our house from day 1 her family was trying to move in here or they were asking if we could hold on to there boxes full of carp for them (which he have and still have them, a whole year later). After about 4 months of living in this house her family came to visit 3 times and each time they would visit for 2 full weeks. Then at the 5-6 months of being in this home now the sister needs a place to stay. I was upset because i couldnt even enjoy my own house. So anyway the sisters stayin with us and the only rule i had for her was i dont want anyone in this house that I dont know because I have exspensive items in my house, if I would be there or my girlfriend they she could. So one day I came home from work and she had a guy over fulling around with him. and i think i didnt mention they are both married, so i kick her out. So after all this when my girlfriend and I were trying to make stuff better between us she becomes all of a sudden great friends with this girl at her work. And out of no where she was going to vegas all the time with her friend without me and palm springs without me which I was okay with. I trusted her but the thing that made me break up with her was one day she was suppose to watch a movie with that new best friend of hers and they did and my girlfriend called me saying they were going to go back to her place and hang out, which I was fine with as well. But she told me she wouldnt be drinking and would be home kinda early. So I goto bed at 11 cause I had to wake up early, then all of a sudden I wake up and its 4am and no call from my girlfriend and shes not home. I call her and no answer. 430am rolls by and im still trying to call her when she answers she says she had some drinks and must of just fell alseep. So now my trust with her is not doin so good. So i put a keystroke program on my computer to see if she is talking to some other guy and not even 20 minutes after I put that progame on the computer she gets on.. After she gets off I go check the program n it showed that she was talking to some dude who she use to know a long time ago in grammer school that just happened to be her new best friends friend and on the note he says it was so nice seeing you again and I had fun playin the drinking games with you. we gotta do this again. and my girlfriend wrote back it was so nice seeing you blah blah and yeah we should do it again. and also on the program it shoulds this picture of my girl under someones arm. So I confront her about this and at first shes tells me that yeah that guy wrote her but she didnt write back, which she did, and that other guy that has his arm around her is just her best friends 18year old bro. Im 23 and now I have to deal with all this. I wish I didnt put myself in this situation. Now i have to live in a house with her because both of our names are on the lease, till we can at least sell it for what we got it for. And the thing that makes it even worse for me is that I put down 100% of the money which was about 10,000 so we could get this house is gonne be gone. Im just so upset. Sorry to babble, if you have any advise please let me know