She sounds like she is fairly immature still. It might be wise to let her go and hope she grows up. However that is a decision only you can make. Having a relationship where you don't feel you can trust the other person isn't very healthy for you. Rather than waste time on worrying whether or not she is being faithful to you, you could be focusing on a relationship with someone else who IS worthy of trust!
Unfortunately, I don't think she is likely to change the behavior she has been engaging in until she has grown up some. The tears that night were most likely a way for her to play with your emotions. (Trust me lots of girls do it!) My personal opinion is that if you don't have trust you don't have a relationship! If you DO want to continue the relationship you HAVE to forgive and forget or it won't work for either of you.
