Advice?

I’ve been with my gf for almost 6 months now. This past weekend she ran into her ex in public(her first love) and I felt she was off for the rest of the week. She didn’t respond as quickly as usual, wasn’t as excited to see me(or at least it felt like) and I just know she was off for a couple days. She has a diary that she usually uses and I noticed it changed places(moved to the middle of her bed) the day after she saw her ex. She left me home alone for a while, and I know this is absolutely horrible of me, but I read her diary. I know this is so incredibly intrusive of me and I feel horrible about it, but the things I read were exactly what I was scared of. She basically said how she misses her ex and thinks about him all the time and talked about some of my flaws and at the end said “i miss my “ex boyfriend” and i wish he would reach out”. I know my gf would never cheat, and I feel like these feelings were just a temporary flashback to her time with him, but I can’t stop thinking about it. Her ex boyfriend treated her like shit and I can’t believe she still thinks this way about him, but I understand she can’t control the way she feels.

I brought up yesterday when we were hanging out that she felt off after seeing her ex and I told her how I felt about it and she kinda just pushed it down(which I don’t expect her to admit her feelings for her ex), and I’m trying to be emotionally mature, but I just can’t get this through my head. Again I know what I did was so wrong, but I did it and I understand the consequences I have to live with now after finding out this info. If you were in my shoes how would you feel about the situation?