A situation much like that happened to me a few years ago. He was doing his second degree as he messed up his first and he didn’t have time to invest in a relationship. Or that is what he told me.
After New years, he tells me out of the blue that he had invited his ex over to ask whether they should get back together. Apparently, he had been thinking about getting into a relationship for a while (and yet this was the first I had heard of it).
Then he goes on to say he was debating about getting into a relationship either with his supervisor at work or his ex. But he decided that his ex would be a better choice as they had history. I felt invisible and the way he told me so brazenly as if I was nothing was hurtful. The guy knocked my confidence for a long time and I started doubting whether I was attractive too. The fact that the guy chose the other girl has no reflection on how attractive you are. To be honest, do you really want to be with someone who treats you like scum? The guy has done you a favour.
The guy used you and he is going to keep on doing it by pushing the boundaries. He doesn’t sound like he cares about you at all, even if he may tell you otherwise. You made a mistake and I’m hoping you will learn from it. If I were you, I’d break ties with him NOW. Stop sleeping with him and have nothing to do with him. Let the guy know that you won’t be disrespected.