I don't trust my girlfriend anymore, but I'm not ready to give up on the relationship?

I worked two full-time jobs and went to school full-time all in the same semester to make a long-distant girlfriend become a not-so-long distant anymore. We live together in an apartment. Everything was going great until she would start getting depressed. And the depression was like an on and off period. Like it would happen once every week, usually when I'm talking to her on the phone when I'm doing nothing at work. I did everything I could to talk to her - to find out what could possibly be wrong, but she always says, 'I don't know what's wrong.' Well, not too long ago, she confessed to me that she texted my cousin, talking about how She and I weren't meant to be together and that she was basically thinking about leaving me, but she doesn't feel like moving back to where she use to live, which was with her parents. Then she told my cousin she had feelings for him and wanted to know if he had feelings for her. When she confessed this all to me, she toned it down as if it wasn't as big a gesture, like a more of a spur of the moment thing. And before I had a chance to read the texts, she deleted them. She told me she deleted them because she was afraid of losing me. She realized what she had done after she had texted my cousin her thoughts. Well, she didn't take into account that my cousin and I are so close we considered each other brothers. He should up at my work and showed me the entire text conversation. The conversation was a lot more in depth than she explained. I broke it off with my girlfriend, and I told her I'd help her set up with an apartment somewhere around here and get back to her parents. She convinced me that she's changed since what she did and that she should have another chance. I told her I would give her another chance and to be straightforward, everything has been pretty good. She's even been giving me very special treatment in the bed. But I have even told her that I don't really trust her anymore and it would take awhile until I get it back. I still love her, but I keep feeling like I just want things to go back to the way they were even though my mind tells me that's impossible. I accessed her facebook account, where she leaves the password on the laptop and found that she was messaging an ex-boyfriend about six months. The ex-boyfriend was definitely the aggressive one in insisting that she missed having sex with him. And even though she was a bit dodgy, she told him that she did miss sex with him 'a little.' This all happened before the texting with the cousin incident. I get the feeling that my girlfriend believes and understands she's in a relationship with me but that she doesn't really understand that what she says and does have consequences and hurts me to hell. So I guess I'm just asking for anyone's take on this?