My current girlfriend and myself have been dating for around 2+1/2 years.
At first, our relationship started off very rocky. She had broken up with a boyfriend/fling she had over the summer (the one before her then Junior year in High School), and we had started talking. We started dating a couple months into both of our Junior Year in High school. About three weeks into the relationship, however, she had a romantic encounter with her previous summer-fling. It wasn't sex, but they had kissed.
This really hurt me, but for some reason, I don't know what it was, I decided to stay. I wasn't as hurt as I would be had this happened a while into the relationship, but it still hurt me, seriously, for a very long time. I never "let it slide" as it were, however I never threw it in her face. Since then, we had moved on, and I had grown to love her.
We have now been dating for 2+1/2 years. Her senior year in high school, she had enlisted in the Marine Corps. to help her pay for college, and to give her additional time to decide what she wanted to do in life, as that would allow her 4 extra years with pay, benefits, and a future pension to fully make up her mind. Since then, she had been through many hardships, boot camp, MCT, her MOS school, and finding out she would be shipped to Iraq in the upcoming months (around September).
At her MOS school, she had made a few friends. Mostly drinking buddies. Although drinking under 21 is illegal, the Corps. turns a blind eye to it, deciding that they are merely kids wishing to have fun. Well, some of her friends from her MOS school have been sent to KAK with her. I don't know what the acronym stands for, however I do know that it is training for going to Iraq/overseas.
Having not been able to talk to her for a while, I found out some news tonight when I talked to her. Judging by the name of the forum, you can probably guess what the news was, unfortunately.
She had cheated on me, once, with a guy that she is friends with. This is a guy that is in the same job field/stationed in same area as her, so she naturally sees him every day, and cannot simply avoid him. Neither of them were drunk at the time, and he came on to her when they were hanging out, and she just went with it. She doesn't know why she did it, and says that it was pure stupidity (in her words).
As it is easter weekend, yesterday was the first day she had access to a phone to call back and talk with me, so I can't blame her for not telling me sooner. Yesterday, we just caught up with how each other's lives were, etc. for the 10 minutes or so that she had to talk with me.
Tonight when we talked, she broke the news. She says that she did something very stupid, and still wants to be with me. In her words, I have every right to be mad at her, and nothing she says can justify what she did. I deserve better, deserve to be happy. (paraphrased from her words)
I don't know what to do. I still want to be with her, I still love her. I don't know how I can trust her, or work at building up our trust as she will be away in Iraq in the upcoming months, surrounded by similar males to the one mentioned. I can't exactly take a break from the relationship, as any news, regardless of how honest, would come as additional hurt to me.
I could never hurt her, especially in the ways that she has me.... but I still want to be with her, even through it all... but I don't know what to do.
I know the proverb, "fool me once, shame on you, fool me twice, shame on me." Well, it seems as if I am the fool in this matter, from the above details...
I don't want to be hurt again, but simply closing myself off to hurt isn't the answer, in my eyes. I've seen what love can do, and I've experienced it with her, and I know that the goods have far outweighed even the bads, but I don't know if I should simply move on based on the sheer principal of the matter.
Am I being overtly foolish by wanting to remain with her? I know I can't have any additional assurance that a similar thing won't happen again, but I don't know what to do, so I'm turning here for advice. I really appreciate that everyone reading this has been through a similar situation, and can relate in more of a personal way than most people that I would ask for advice otherwise.
Apologies for my novella, but I thought it only fitting that any details be given before any judgement can be made in any way

If you are able to, please try and provide detailed answers, not just a "go for it" or "dump her" type of a thing... but if I should stay with her, anything that could make it work-- or any way I should try and build up trust in her again, etc. or likewise, reasons I should dump her.
Cheers, and thank you in advance for your help
-David