<human>: I know I'm young and a lot of you think this probably doesn't matter but holy shit is this getting to me.

I'm totally blindsided by this. I don't know what to do. We had what I thought was a very happy relationship. We texted, called, and hung out often. She was always happy to come to my house and we'd watch movies, play video games, cuddle, stuff like that..

She was honestly my best friend, especially after me and my previous best friend had a falling out. Since I've felt very lost, and honestly scared to get another best friend.

Anyway, today we were hanging out and when she went to the bathroom I picked up her phone. I just wanted to read through our own text conversation (I don't know. I thought it'd be cute.) but the top conversation wasn't with me. Confused, as I barely knew the guy I opened it up just to check what was up.

The last text was saying "I can't wait to see you again. <3" My heart sank. I honestly just locked up. I scrolled through it a bit and saw that she was sending this guy explicit pictures too. I put her phone down and pretended nothing happened, but she could tell something was wrong. I didn't tell her, and she still doesn't know I saw.

I really don't know what to do. I know I can't keep dating her. I don't know why she was so fucking obvious about it and didn't even try and hide it. I lost everything to her and this feels like such a stab in the back. I know this is so cliche and exaggerated but I just don't know what I'd do without her. Without her I don't really have any actual friends.

I just want advice. I know most of you are going to blow this off because of my age, I just didn't know what else to do.

Summarize the given text in a few sentences.
<bot>: Found out girlfriend was cheating on me, and it's completely not expected. She doesn't know I know and I don't know what to do.