Your boyfriend is not the issue. You are. That girl hasn't done anything to justify these thoughts and your boyfriend certainly hasn't done anything to warrant being told that you do not trust him.

You need to continue therapy and probably need to be single until you learn how to manage your emotions and your control issues. You can not control people.

>sometimes I’m worried that he’ll actually start liking them and leave me for something he likes better (which I think is fine but I would just want him to tell me asap and not hide it and feel guilty or something and keep dating me because he feels bad for me).

This part right here is disordered thinking. If a person actually ever does cheat on you, you need to have enough self worth to walk away. Saying that your main concern is that they would keep dating you, because they feel bad for you is messed up. Find your self respect and love yourself enough to know how to handle yourself and be secure within yourself.

Your worth does not come from a man or a relationship.