I think if they're toxic, they have a way of sucking you back in when they hurt you. It's the cycle of abuse thing, where they're caring, then hurt you, then make up for it and things seem ok again, until they hurt you again.

Or it can be much less overt. Where it could be they just have really inconsistent boundaries. So it's confusing. But can also draw you in. The intermittent reinforcement thing--I had that with my former marriage counselor. Where sometimes he'd reply to my emails or texts, and sometimes he wouldn't. But that made the times he replied seem more meaningful. Sometimes he'd talk to me (individually) on the phone or let me talk about my own stuff in a marriage counseling session. Other times, he'd say we have to stick to marital topics.

It was difficult to leave him, even when we had a major rupture where he really hurt me. It still went on for 4 months, where we kept going, and I kept hoping we could work through it, until finally I realized we couldn't.

Ex-T, I had some issues with her, but she was generally good/consistent with boundaries. With her, I decided to try out another T, saying I'd go for a couple months then come back or else if I opted to stay with him, I'd at least return for a termination session or two. I never returned. Leaving was easy.

Current T is more complicated. He generally has better boundaries that ex-MC did (though less so since the pandemic), and I stayed through a couple ruptures. Then we had a major rupture, and I left, saw another therapist for 2 weeks, then...went back to my current T. So I guess it was difficult to truly leave him, too, but it was more that I felt like he could still truly help me (and he has since then). Rather than being unable to leave for other reasons.

I know that was mostly rambling about my own experiences, but hope it gives you some sort of insight. I just think there can be something about inconsistencies and being unsure what you'll get--assuming it's good some of the time--that can draw a person in (I've experienced that in past romantic relationships as well).