Hmm~
I guess, try to understand it from different angles for both parties.
The good thing is, both of you are sharing and sorting out things together. It is a good thing that your wife is sharing her past experiences with you, regardless of who is bringing it up. The bad thing i guess, is that you can't put it out of your mind.

As you have written, prayer has helped you. I don't know if this works, but you might consider trying it out. Try asking God to help you forgive her, and release the tension from your mind. Then slowly meditate on the good times you have had together(after all, you have been married for 13 years!), treasure her and understand that what has passed is the past.

Also, try to talk to her if you feel uneasy about it. Communication is vital in any relationship, regardless of age.

Hope i'm not repeating anything. And also, good luck in your relationship.