My (25M) Gf (25F) writes about another guy on her diary. How do I proceed?

My gf “Mary” and I worked in a lab; she’s an engineer, and I was in admin.

There were several of us who were about the same age, including “Peter” from another department. I worked outside Maria’s department (open offices kinda space) and I always saw Peter going in and out

Like a year ago some of us were on a bar and, one of the girls let slip that there were rumours that Peter had been going to Mary’s department just to see her and talk to her. Her coworkers agreed that there was always too much tension between them, that he sometimes brought her candy or soda, and that once he stayed unpaid overtime just to keep her company.

I never felt that tension or saw those signs, but I do remember feeling really bad, because I’d liked Mary since we were in college. I’ve always been bad at love, and it took me a long time to confess how I felt. Plus, ever since that day, I’d felt insecure that so many people thought they liked each other.

I quit in march, and in May I confessed to her and we’ve been together for six months. I wanted to suggest we live together until what I saw yesterday.

Last night, she had an anxiety attack; she got really bad, I tried to calm her down, but she locked herself in her room for about an hour. When she came out I made her some food and asked if she wanted to be alone, and she said no, that she’d locked herself in because her therapist had recommended that she write in a diary whenever she has very strong feelings.

Before we went to sleep, she received a message from Peter . She didn’t reply to him and fell asleep pretty quickly bc she’d taken sleeping pills. i tried to guess her phone password but couldn’t, so I looked for her diary and found it under some books at the bottom of her desk.

It was full to the brim, and it was one of those thick, 200ish page notebooks. None of the entries were dated, but I pieced together a timeline based on what I was reading. I found the diary entry marking the dates when we started dating and ended up satisfied with what I’d found, but I was looking for Peter.

It was still relatively early on. The diary entries said that she didn’t know how to feel, that she’d realised she got excited for lunchtime because she knew they’d run into each other, but that it wasn’t a good thing since they worked so close together and could jeopardise their jobs. She felt like a fool because he probably was just being nice to everyone. There was even an entry where she was disappointed that she hadn’t been his secret santa last Christmas

This is emotional cheating right? I don’t know how to confront her, she could be physically cheating on me with Peter and I don’t know what to do.

English is not my first language, sorry any mistakes