Ok, i'd been dating this girl for over a year, and we broke up in may when she graduated (we go to the same college, I'm a year younger) because she wanted space or whatever. At the same time, she was playing bass for this band and was confident (especially after she told me it was true) that it was because she liked the singer of the band.
Less than a week later, we start dating again, with her citing a momentary lapse in judgement and me figuring nothing of it.

That was two months ago. (she's still in that band, but has assured me that its over with them)

Today, she was over at my house, after she left I popped on my pc and found that she left her gmail account open. The entire inbox is filled with messages from him up until about june 20, each with pages and pages of them keeping in contact and very obviously fucking and having a relationship behind my back).

I called her, furiously, tonight.

I have done everything I could to be there for her for our relationship, but now, even against my better judgement, i told her i'd give her another chance if she promised to quit the band and never talk to him again. Is this reasonable? Am I too forgiving? Any thoughts?

you're a tool. Leave her and steal some of her shit while your at it.

Fuck her. What is it you see in someone who will cheat on you without feeling guilty. This is a long time, and if she was remorseful she would have fessed up earlier.

Fubar-- Hmm, not quite what I was looking for, but surely qualifies under "Any thoughts?" Not real big on stealing but this clearly counts as one for the "dump her ass" vote.

Apocalypse- You basically represent one half of my brain right now, you know, that logical side that makes feasable decisions, and I feel like a total dumbshit for not being able to yet follow that, but I'll try.

ditch her... once a cheater always a cheater might be a little different if it was just once but if she was fucking him for awhile dump her and don't be nice about it.

That seems to be my current direction...thanks

I would leave and her not think twice about it.

In a relationship the couple should be completely open, trusting and honest with eachother. With all that gone, what else is there really?

I would move on and find someone else who wouldnt betray me in the worst way possible.

Lying, sex with others, lack of trust: that is no way for a relationship to thrive.

Call that British radio show where they dump people on the air.