Oh christ.. If this doesn't bring back memories of the last couple of years of my life.. Been there and done that myself, but then again, I'm an optimist and always will be.. and I've been in some fucked up situations. Ok so here's what happened to me - different, a tad more extreme, but just bear with me.
Firstly, my ex had some really well planted issues going down which I have no intention of trying to explain but yes, she was more than prone to sleeping around and yes, I tried (in vain) to try and change her ways. Not that I'm saying I was or am perfect.. but God did I try to understand her and make things work.
I met my ex-girlfriend online. Were just friends for a while and kept it at that for a while. Then she decides i'd be a far better match that her bf at the time and we decide to try our hands at dating. (Mistake number one)
Kept the long distance thing going for bout 6 - 8 months and met up just before christmas. I had started uni that summer in March and she was in her last year of high school. We had all these plans for the future and fell quite deeply in love.
We maintained a high level of openness (or at least i did) and had little things go awry (like her making out with a girlfriend and random shit like that) but I kept the faith and we worked everything through.. somehow.
Fast forward to july that year and she spent a week at my flat (like a room in college, but a whole house where you have flatmates as opposed to roomates). Anyway, pretty life changing stuff for me and it bought a whole new dimension to our relationship. And a whole lot more shit. All this emotional baggage and doubt from her came up on the last night she stayed with me and somehow I worked through all that too and we carried on as strong as ever.
A few months later she wanted to break up with me, I tryed to find out why and asked her to stay with me (what was I smoking?) I find out later that she was hiding something from me. When I confronted her it turned out she'd cheated on me twice, once while drunk and once with a guy who I could feel something was going on but I didn't let jealousy get the best of me and ignored it.. (Here's comes mistakes 5 or 6 through to about 978)
I stayed with her despite everyone's advice. I even lied to one of my best friends about what had happened. I bought her regret, her apologies and everything else. And somehow she gained my trust back..
Then, she has a huge fight with my mother around christmas, constantly drained every part of my soul away with her irrational jealousies, trust issues, and baggage from past relationships, while I try my best to be as supportive of her as humanly possible.. and finally SHE dumps ME for some random guy of the street who asks her out one day while at work. What's worse is her mother calls me to let me know what's going on
Great.. Hang on.. I may have already posted this up at some stage. At any rate that was the end of that and thus concluded the longest mind fuck i'd ever experienced in my life. Thank god for that.
So anyway. She gained my trust despite all the messed up crap she'd done, I believed her words and continued to be as loving and supportive as always. She turned around and dumped me anyway.
My advice: I know we have/had different situations, starting points and relationships. I'm not going to say break up with her and i'm not going to say that making out with a guy for 10 minutes while drunk is less serious than going all the way while drunk. If you seriously feel you can forgive her, then by all means forgive her. Just make sure nothing and I mean nothing like this happens again. It all comes down to control and if she can't control it, she may never change and you may have to move on. There's always misery in life but it's the rewards which make us take the risks. And a there's nothing more rewarding than a loving long term relationship. I wish you well with whatever you chose