It annoys me tbf he's done this behaviour for years and he walks away with out a scratch...
So have you. You've slept with him on and off, knowing that the majority of the time, he is with someone else.
He will walk away from this again, because you look just as bad as he does. You are not some wounded party. Your pride is wounded, sure, because he's blocked you and walked away.
And if, by some miracle, his new girlfriend didn't fall for the usual "it was before you", "she's obsessed with me" rubbish, you're hoping that he'd run back to you. Maybe he would, but it would still be for sex and nothing more, you would still give in and have sex with him in the misguided belief that he must like you if he's showing you any attention at all, and he'd still go and find someone else to actually date.
Rather than giving him so much headspace and fantasizing about telling his girlfriend, consider how you've ended up in this position and how you can stop it happening again. All the bumpf about how you're going to university and starting a new life, how attractive you are, how much better you can do has not sunk in. You don't believe it, you're just spouting it like a mantra. Work on that.
There's no "throwing a lit match and watching it start an inferno" scene here. This doesn't end with you walking away as the brave heroine and him being single and sad and alone. You're going to look just as bad as him, but he's more experienced and is at least owning what he did wrong, so ironically, he'll probably come out better and it'll all backfire, leaving you looking worse.