Moral dilemma--not sure what to do

Hi everyone I've read around gamingforce a lot but never posted before... until now. I need advice and really don't wanna bring this up with a friend.

Me and my girlfriend Andrea dated for a long while. We had an amazing relationship which then turned sour about a year ago. We were going out until around halloween were we decided to take a break which then only became a gradual decline into us becoming completely separated in the following months.

While I was in the process of breaking up, Andrea was spending lots of time with her new super best friend who was a lesbian. I knew that they were attracted to each other (andrea is bi), and also knew that they ended up together at the same time Andrea and I went on a break. I completely trusted Andrea and thought that she waited until we broke up to mess around with her new girl.

Well it turns out that my copmuter saves IM conversations and I happened to be snooping around today. I read one of their conversations, and discovered that they messed around a whole bunch a whole three months before we "took a break" and were madly in love with each other. It sickened me.

So now I know that my ex cheated on me. Great. Because right now we are trying to mend our friendship after some rocky post-breakup months and I just want to get things behind me. Well now I'm fucking pissed that she cheated on me (a whole year ago mind you) but dont know what to do. I pretty much have two options that I can see.

1. Suck it up and forget about it which leaves me unhappy.
2. Tell her that I know she cheated (which I'd love to do), but that also means I tell her that I read one of her conversations. I will have breached her trust, but at the same time she will have nothing on me because she fucking cheated on me.

Maybe there's another alternative. Help anyone?

Jam it back in, in the dark.

Thanks for your advice everyone... particularly blue.

I'm leaning right now toward doing something like having a serious talk with her and telling her that I have something on my mind that's been bothering me for a while. I'll then ask her if she ever cheated on me. Either way, I'll tell her the truth afterwards about what I've done and man... if she lies that will be interesting. Hell if she tells the truth though that will be very strong of her. Hmm.

Thanks again!

There's nowhere I can't reach.

If a clean break happens, it happens. I'll let time decide.

I still refuse to let her think that she got away with doing such a thing without me knowing.

This thing is sticky, and I don't like it. I don't appreciate it.