I feel cheated

I don't exactly know how to deal with this relationship and my trust problems anymore. What happened was that a few months ago I found out that my boyfriend had a crush on a girl from work (she no longer works there) and they had been texting. When confronted with it, he admitted it and told her they could no longer be friends. I had this strange suspicion that something was going on so I checked his phone this past weekend.... he had talked to twice in the week. I immediately confronted him and he apologized and said it was because he heard she was coming back to work and wanted to lay ground rules. It also turns out that they have been friends on facebook. He deleted her friendship and phone number. He gave me his passwords to his cell phone account. I looked last night and saw that he was possibly texting her just last night and the night before. He denies that it was her but refused to tell whose number it was saying that it was too much. I have a 4 year old daughter who is absolutely in love with him and I don't know what to do! I don't believe him. He's been consistently lying to me. He is starting therapy to deal with his insecurities and I just don't know where to go or what to do with myself. I stupidly feel bad for him because he feels so horrible about the whole situation. I know it had to be her last night. When looking at the phone calls from the week before, that number matches the calls I saw on his phone. I don't want to break my daughter’s heart again with another man leaving her life. I love this man. I don't know how to get past this or to trust. I don't know where to go from here! Obviously don't want to talk to any friends or family about this. Someone please give me some guidance. I'm desperate.