How do I talk to a girl(21F) if I(22M) secretly read her correspondence and suspect cheating?

It’s a long story, but I really need some advice. I would appreciate if you could read it and give me some guidance.
We have been together for 5.5 years. My girlfriend has depression and is taking pills as prescribed by her doctor. This is just to get a general understanding of the picture.

On the 5th of May she went to the party, she stayed there late and wrote that some shit had happened and that she would tell me later, and after midnight she wrote that I shouldn’t wait for her because she was staying with a colleague (she didn’t specify male or female). I didn’t think anything wrong.

After that I decided to go to my parents in another city for a day, I decided to leave on the 8th and come back on the 9th. She didn’t mind, and then she even asked me to stay with my parents a little longer and come back on the 10th. I was a little worried because she was depressed, but she said she was fine and she just wanted to be alone. I didn’t mind either and came home later.

On the 11th, I was driving home from work and she asked me to buy cigarettes and wine, she said she was on a binge. I didn’t really understand what she meant and it didn’t seem like a good idea to mix antidepressants with alcohol, but I bought what I needed and came home.

She was sitting on the balcony in a T-shirt and a blanket, it was cold outside. She had finished her first bottle of wine and was sobbing. She was screaming and bursting into hysterical laughter. I went over to hug her, she hugged me crying and apologizing for scaring me. But she couldn’t calm down. And she said she would explain everything to me later.

She said she called her friends and colleagues to help her, but she wouldn’t tell me what happened or why she felt so bad. I also saw a lot of fresh cuts on her leg. (I understood from talking to her friend that she made the cut on the 10th or 9th).

When her friends arrived they were able to calm her down a bit, get her into the apartment and talk to her. She asked me to make dinner, so I did that while they were talking in the next room.

From her friends’ conversations, I was able to pick up something about a male colleague (let’s call him K). She said he was an asshole for driving her to this state. She said that if I had stayed late at work she could have jumped out the window by now.

But they didn’t say anything specific, and then I heard her friends say something like “she likes K and he’s doing this to her.” I didn’t think much of it, and then she called K over to our house. He came over and they talked for an hour in a closed room without everyone else. While I was cooking food and her friends were discussing something.

Eventually everyone left and she fell asleep almost immediately. From that day on, she started talking to me less and her answers to all her questions were very dry. No emotion or slightly irritable. But I tried not to pester her, because she was having a hard time and I understood that. And then on the weekend of the 14th-15th she went alone to a hotel with a pool to rest, it is not far from the city and she has long wanted to be there.

And so Saturday morning while she was asleep I decided that I wanted to figure out what happened, and I thought that in the correspondence this K might have been writing something to her, maybe apologizing or something like that. I thought I would find a hint of what was going on there. Even though I realize that’s not a good thing to do.

But their correspondence (besides the work one) started just the morning of May 6, after the party, and she wrote him “I hope that everything will stay just between us.

Then the correspondence looked like this

C – I’m walking around covered in bruises.

G – if they’re on my back, it’s me.

G – and my lips are all covered in little wounds.

K, I’m sorry, it’s me.

K – I’ve got a scratch on my neck that looks like a hickey.

G – maybe it’s the forest, maybe it’s me

G – and my back hurts after your massages, when can I make an appointment for the next session?

then something distracted, they agreed to meet after work, and i she took some wine from home that day, said she had a meeting (but did not say with whom). it was may 7

And then she arranged for him to come to her place for the whole day on May 9, she said “I’ll be waiting at my place on 9, the most perfect day.”

This correspondence is after she told me that she wanted to be alone and asked to stay at her parents for a longer time

Anyway, I don’t know what I should do. I’m not sure if they had something, but it’s all fucking weird. And I feel really bad about it. But how am I supposed to talk to her about it? She never said anything about anything, even though she said 2 times that she would explain later.

Should I tell her that I read the correspondence, or just ask her what kind of situation happened with K to see if she will lie to me. Or don’t say anything. I was thinking of continuing to read the correspondence to find more hard evidence. I’m afraid that if I start this conversation, it will turn out that they didn’t have anything and I’m just a jealous asshole.

Thank you, if you’ve finished reading, maybe you can leave some comments to help me. I’m feeling really fucked up right now.

TLDR: I secretly read the girl’s correspondence because I wanted to make sense of the situation. But I found some strange things there and now I think she cheated on me, but I don’t know how to talk to her about it.