I used unethical means to learn she lied (and "cheated"). Advice?

10 months ago, I started seeing an old girlfriend again after three years apart. We both weren’t looking for a relationship but we got along well and the sex was great, so we decided to be FWB while she dated other people. I was okay with this arrangement with one caveat: that she let me know if she slept with someone else (we both were only comfortable in a monogamous arrangement). She agreed that she would let me know beforehand if she felt she was about to get into something serious with someone else.

As these things usually go, our feelings for each other grew and the arrangement became complicated (causing a few fights and reconciliations) and we sometimes talked about going exclusive. I was strongly considering going steady with her. Recently, she left a tab open on my computer, logged into an online diary. I couldn’t help myself and read through her entries. I learned that she had slept with one of the guys she was dating less than two weeks ago. I was both devastated and thoroughly grossed out.

I am upset that she slept with someone but feel I am in the wrong because I should have seen it coming. Our arrangement, after all, did allow for that (we weren’t in a committed relationship). But what really hurt was that a woman I have known to always be blunt and tactlessly truthful… lied to me. I wouldn’t have believed she was capable of it if not for the evidence in front of me. She slept with me after being with someone else, despite promising me not to do that. She knew this was a big deal for me.

I am ashamed of the fact that I broke her trust in reading her diary, but at the same time, consider myself lucky that I found out this information before committing myself to her. She is now no longer dating anyone else and wants a real relationship with me. I turned her down without explaining why. If I had never read that diary, I would be with her now!

So now I’m in this weird place where I’m still sort of in love with her, but know I can’t trust her. I’ve tried probing her, but it doesn’t look like she is going to come clean. Despite her betrayal, I still care about her and don’t want to lose the friendship we have (I’m not interested in having sex with her since learning about what she did). She’s one of the best friends I’ve ever had. However, she doesn’t seem to be interested in a friendship because she has feelings for me.

I don’t really know what I’m looking for in response to my story, but I think I could use some outside opinions and insights to help me sort out this mess. I am deeply hurt by the way things turned out. Do I confront her about what she did and how I found out? It will definitely end the friendship and I admit I don’t want to do that. But it seems like the friendship might end anyway because she doesn’t want it.