I fully took it as a mistake after we talked about it. I have been telling myself she made a mistake and she says she won't do it ever again, so I need to trust her.

And I don't mind all of the extra questions at all. There wasn't really a relationship with this guy. She says they were "together" and would sleep together and hang out, but he had other girls on the side, which she knew about. I went into her email to actually check on something else, but seen an email from him so it peaked my interest. I know it was bad of me, but I am still in the "gaining trust back" stage. If her friend hadn't told me, I honestly don't know if she would have. She said she was afraid to tell me in the first place because she didn't want to lose me and she wouldn't know what to do without me. She continues to tell me that she wouldn't know what to do without me, which makes me wonder why she would still talk to him behind my back knowing how I feel about it.