Can you trust after your trust was broken?

Ive been with my gf for a while now, in the beginning she cheated on me with her ex. I forgave her and continued the relationship, trying to move on. She gets mad because I find it hard to place all my trust in her. Recently, I have noticed her ex still leaves little love notes to her as his myspace heading. In fact his name on there is "Thinking of my ex T.S.B." before, he used to try to contact her through all online sites he knew of yet hasnt for some time. Should I still think they are talking?