Girlfriend broke up with me, do I need to know the truth?

Sorry this is so long!...
My girlfriend of 3 years broke up with me 3 weeks ago. I was completely surprised and devestated. We were very close the whole time we had been together, and we were madly in love. About 2 months before we broke up, she met a guy who I was immediately suspicious of. They met in the library (we are both students) and went for a drink. She was honest with me about this so I didn't worry about it. She just said they got on well and were friends.
I accepted this, but I warned her that she should be careful, because guys don't usually ask random girls out for drinks just to be friends! She has always been a bit naive in this sense.
Anyway, we carried on and I knew she saw a lot of this guy. I got a little suspicious but I trusted here. Then after a while when I asked what she had been up to I could tell she was lying to me. She'd quickly change subject when I asked about this guy and she didn't seem as comitted or loving towards me as she had been. I asked her several times what was wrong. I gave her many opportunities to tell me what she was thinking and I told her I knew something was on her mind. She said everything was ok to start with, then one day she told me she was having doubts about our future. She said she wasn't sure we were outgoing enough, which annoyed me as she had glandular fever so I thought was unreasonable.

Moving on, when she told me this, I said that we could try to do more things individually as well as together, and that I didn't see it as a problem that couldn't be solved. She agreed. At first I felt like a weight had been lifted, but I soon realised that this wasn't really what had been on her mind. I continued to ask her what the problem was but she said there was no problem. Then, one day, a friend told me he'd seen her with this guy in the library having lunch every day and studying together. I confronted her and she said there was nothing going on. She then got overly protective of her phone. By this point I was convinced she was lying to me. I justified it myself to read through her messages. I know it was wrong but I felt I'd given her every opportunity to tell me the truth.

There were a lot from this guy, and he had openly confessed that he was falling in love with her. The fact she hadn't told me this hurt a lot, but I trusted that she wouldn't cheat on me. I gave her another opportunity to tell me about this guy's feelings for her and again she refused. So I admitted to reading her messages and I knew he was into her. We spoke for hours about it, and she said she didn't want to worry me and that she had no feelings for him. She offered to stop contacting him and I accepted, thinking that would solve any problems.

This time I felt like a huge weight had been lifted, and that we could return to how out relationship had been previously. However, as time went on, she would see him now and again. I didn't mind the occasional bumping into each other so I didn't say anything. With time, she seemed to see him more and more, and when I asked, she said that she thought I didn't mind. I told her that I didn't want to stop her having a social life but that I thought seeing this guy would damage our relationship. Anyway, we were getting on well again, and then she broke up with me. She said it was because we were too similar, not outgoing enough, etc. I was completely heart broken. The first two weeks were horrible, I couldn't believe she would end our relationship without giving it more of a chance. We hadn't spent much time together recently due to exams, and she broke up with me just as we had time to see each other.

This was around a month ago. A few days ago, I went onto hotmail on my computer(my account that I hardly ever use) and she had saved her password from when she had used it months ago. My curiosity got the better of me, and I read some of her incoming facebook messages (Icouldn't see anyting she had sent). Anyway, there were some from a mutual friend of ours, which said that he had told her that he liked her (my ex) but that she had decided she wanted to be with this library guy. There was no mention of me being considered in any of this, as if she had been weighing up these two other guys (one of which was a friend of mine who I trusted)for a while, without considering me into the equation. An email from this other guy saud that he was missing her too etc.

This completely devestated me and I don't know what to do. Should I call her and ask her to tell me the truth and the real reasons she broke up with me? We both said we wanted to stay friends, and we've spoken a couple of times since we broke up, but I feel this changes everything. I am not sure I could be friends with someone who could do this. What really hurts is that she knew I was having a horrible time coming to terms with the break up, and that I couldn't understand why she had done it without giving me a chance, yet she just left me to try and get my head around it when she was lying the whole time.

Sorry for ranting so much, its pretty complicated. I don't know whether to confront her and find out the truth (which means admitting I read her emails, and may jeapordise any chance of friendship, as well as hurting a lot), or to ignore all this and just pretend it didn't happen?
Any advice? Thank you!