I got cheated on.. and I stayed. Advice to let go?

Girls, i know I'm not the only one who's been through this. I was looking for something out of pure curiosity and I found it, I guess I deserved it too. 2 days ago my boyfriend left his account there on messenger, but you had to use a password to get in ofc. Since I know him like the back of my hand I guessed his password and was so surprised when I got in, but I didn't feel bad since he's had my password before. I immediately got a bad vibe but I decided to click on his GIRL best friends convo, which I never really minded her since she's practically "one of the guys". Let's just say there's nothing between them but they definitely do talk about girls, since she's bisexual herself. I saw so many things, I couldn't even bare to move on, I was so hurt. I thought he was the one I wanted to spend the rest of my life with and then this comes up, and it's not the first time.. ofc everyone's telling me I deserve better and to leave him right away but how I seriously cannot do that. I've tried and tried but all I do is run back, I'm literally one of those girls who think they're world will come crashing down without them and I absolutely HATE that about me. I don't even know what to do, I feel like no one understands.. do any of you have advice for how to accept and move on?😔