How do I stop myself from going back into a depressive cycle?

You’re calling her sleeping with some guy she’s barely been with a relationship? Uhm sorry, but that’s not a relationship. It’s a whole lot easier for girls to get with guys who just want to sleep with them. Seriously, any girl can find a guy for that. Also, this is usually just for comfort sake, but in this case, perhaps you’re better off? You’re turning 15? So since you were 12 this has been such an issue it is causing severe depression? Look, I think you’re attributing the depression to this relationship issue, but that is never what depression is about. You may think that’s also the solution, but it really isnt. You cant be in a relationship just to not be depressed. You probably should consider talking to someone about this stuff. Like a professional. Just to help you kinda get up again (it’s hard when your self esteem has taken a blow). And not that I’m advocating self harm, I honestly suggest you find healthier coping tools, but one of the girls I work with is completely scarred up, and that doesnt bother her girlfriend. So dont make it the repository of all your troubles. Whatever is going on is more than about your scars or your relationship status. And you probably want to fix that.