I think my girlfriend may be cheating on me-
Saw an e-mail in her inbox that was suspicious
But I didn't open it because I want to respect her privacy
And up until now I've always trusted her 100%
But now I feel like I can't really be sure
Should I confront her?
If I do, and she's not cheating, then it'll really upset her that I'm questioning her.
But I worry that if I wait it out and find out down the road, it'll be too late to try to patch things up between us.
We've been together over three years
Live together
And she's never given me a reason not to trust her in the past.
Should I just let it go? Wait and see?
Hint at it somehow?
Please help.

What did the suspicious email say? Has she given you other reasons not to trust her aside from the email?

It was a response to a posting from an online discussion board where people meet who are interested in each other. That doesn't necessarily indicate she was one of the people involved with ea.other- she could have just been responding to something someone else posted, not necessarily to meet someone else, which is what I want to think.
But... I saw it in her inbox when I looked at the comp. and didn't open it- then it started eating at me, and I figured I'd see if she read it, which she did. So it automatically deleted. But it wasn't in her 'old mail' folder, which is where things normally go once they're read. And it wasn't in the 'trash'- which means she deleted it and then emptied her trash.
I don't know if that's what she normally does, or if I should see that as suspicious.
I love her so much and don't want to accuse her of anything that might not be true. She's never given me any other reason not to trust her...

Don't confront. Accusing, nagging and so forth will only make her hide things more IF she is.
The best way to handle it is time and patience and watch for signs (red flags). In other words if you give her enough rope the truth will surface on its own.

One sure sign is if things between you start changing. She starts accusing you of things like looking at other girls, talking to other girls, things like that. Or if she's shady about where she's going, who she's with. If nothing seems out of the ordinary things are probably OK, but you want to watch out for signs.

Yea, I guess that's probably the best way-
She has accused me (wrongly) of being interested in other girls... but that was a few months ago and hasn't happened again since then.
But whenever we fight, especially recently, she's been saying things like how she hates me, she's not in love with me anymore, how 'it's over' and we're 'breaking up'- within an hour tops she takes it all back, apologizes, and says she's said it out of anger-
And I know she has a temper, so I usually just roll with it and don't take it to mean anything else-
But now I'm questioning that too.
And I feel terrible about it.
Like... there's something wrong with me for not just trusting her.
I always have and, aside from this, never had a reason to even begin to think otherwise.
So I guess I'll just let it go for now.
Do you think it'd be terrible and sneaky of me to check her e-mail every so often w/o her permission?
Probably a bad idea... right?

She may be just in the 'looking' stage on the site and willing to stay with you for convenience sake until she finds *better*
All you can do is break up or ride it out and see where it goes.
Keep looking for the red flags so you are prepared.