Its going to take time, I've been through somthing like this Its going to take a wile for you to trust her and she needs to understand that. She needs to be willing to deal with you asking questions for your own trust.. But you also have to learn to trust if she is where she says she is and no contact is made then you need to get better and be willing to trust again... I went through this, I set up an on line account and checked his phone for text or calls from the # I knew was her. I could see how many they had in the past which was alot. and then if there were any then. I did see a few coming from her phone but no going out or one or two. I found out when he broke it off she tried to get a hold of him but he just kept telling her to leave him alone. which was verified by all the ones she was sending to him and none going out. That made me feel a little better then till there were none at all. I did this with out my husbands knowledge because I had to know if i was being told the truth. I did tell him after and had to let him know because that wasn't very honest either, but it did make me feel better. When i told him i could tell he didn't like it, but he could see how much it helped me through it. It took me along time to really not let my mind go crazy if he went to the store or whatever..
You just have to be willing to let her earn the trust and she has to be willing to let you get what you need to trust her. hope this help I know its hard!
[edit] I used the att website, also i think you should try to save it before giving up so you can tell your kids you did all that you knew how to do to save your relationsip when their older.