Yours is a situation that's hard for me to relate to because in general I've never really bonded with a romantic partner, so I was always able to leave them very easily and never really thought about them again. However, although it's not identical, I work in a very very toxic office where there are some really painful issues between me and my boss and some coworkers. It's been devastating and bad for my health and a friend said the other day that I've cried more because of these coworkers than she cried over her ex-boyfriend.

Can you figure out why you really can't leave him? I have really tangible reasons, some logical and some irrational/emotional, why I can't go get a new job right now. So I feel very trapped, but regarding my career, I simply cannot leave this group right now. Despite everything, my boss really supports me and wants me to do well in my career and I want to wrap up some important projects and establish myself in the field. Leaving now would be a huge sacrifice, even though I hate working there. What does this guy of yours provide you that's so precious and irreplaceable? Maybe you have some fear that stops you from ending it, or you feel dependent on him in some profound way?

It may also be easier leaving him when you feel more secure, for instance have an exit strategy you're comfortable with. But I'm just speculating.