Can i trust again?

About 2 weeks ago i found an email what my wife sent to her ex-bf. She wrote down she wants to make a love with him and wants to kissing with him all night. When i found that email i spoke with my wife. She said she felt something but it's gone. She says they haven't had sex since they broke up (few years ago) and never cheated on me. I wanted to divorce. She was crying because she wants to be with me as she loves me. I want to trust her but i can't. How can i trust her again ? How can i be sure she won't meet with him anymore and she won't cheat on me ?