What's done is done so stop beating yourself up about it. Reading her diaries was a horrible thing to do, but you can't go back and undo it. Just make sure you NEVER do it again!!!
You definitely need to get this jealousy / insecurity under control or you WILL lose her. You're going to have to trust her that you are the only one she wants just as she must trust you not to be fooling around with anyone else.
If you can't get this under control yourself, try counseling. Good luck to you.