[donny]

My girlfriend and I were in a reasonable relationship and then brokeup for about 2 weeks.
While we were separated she gets drunk and meets a guy, then meets him twice and then sleeps with him because he was "nice" and "paid for the dates" and there was "no pressure" etc. Firstly I think this is a little soon after a breakup and can't help thinking less of her because of it and less of the way she felt about me.
Well we did get back together and she just told me she went to the movies with some guy and I didn't bother asking more (STUPID).
A month later getting very serious, love u love u etc etc, enjoying moments that seemed very special. I even took her interstate to meet the parents and went away together doing nice things. Luckily when we were talking that night away we mentioned the guy she went to the movies with and after interrogation I find out she slept with him and never told me before getting back together.
Now I feel like this last month has all been a lie, I could never have such happy times with someone a distrustworthy to not tell me something like that before getting back together.
The other problem is I can't get the picture out of my head of some guy with her (she described what he looked like) and keep thinking about the details.
I can forgive her for doing the dirty when we were apart, because we WERE separated even though it is a little trashy. BUT I don't know if I can forgive her holding it back. The hard part is I don't know if I'll ever be able to TRUST her again and I've now become a very jealous person and wasn't like that before and always worried about what she's done.
If I stay with her I'll be worried about what she's doing and if I can trust her all day long and I don't know if I can take it.
I also found out that after our very first date right at the start, she saw (and slept with) the guy she had been dating a few weeks before, ie we overlapped a bit. This pissed me off also.
What to do? Any advice?