This happened to me with both my ex-marriage counselor and my current T.

Ex-MC: He had been extremely accepting of me and anything I shared with him, including some strong (mostly paternal) transference. He also said things that suggested he would never abandon me. At one point, a few years into seeing him, I sent him an email saying I loved him (which I'd said before, and he'd seemed OK with), and he initially said it was OK, but that it was a big deal, and I should talk about it with him or my current T (who I'd only been seeing a few months at that point). I said I wanted to talk about it with ex-MC, but he then was saying I should really talk about it with my T, not him.

It led to a somewhat contentious text exchange that led to him calling me and saying, "I only have a few minutes, and I'm going to do all the talking." He was very harsh on the call (which lasted longer than a few minutes), and at the end, he said I needed to reduce contact with him. That crushed me. Sure, it wasn't completely abandoning me, but it felt incredibly painful. He wouldn't even let me talk to him on the phone about it a few weeks later without my husband chaperoning (he used to be fine with phone calls just with me).

I kept trying for a bit, with H and I still going to sessions, though less frequently. I ultimately just realized that I couldn't trust him anymore. And that looking into his eyes didn't have the same calming effect that it used to have. It was like things had shifted, and there was no going back. So we terminated. I've had a few email exchanges with him since, and things are generally amicable. But I don't think I could ever trust him in the same way again.

I feel that the reason we couldn't work through it is that I saw this different side of him during the phone call. And his telling me we I had to reduce contact felt like abandonment. I lost my ability to trust him. He was no longer the safe, accepting paternal figure that I thought he was. And it felt like my love was poison. So I just couldn't continue.

Separate post on current T (spoiler alert: we worked things out) in a bit.

ETA: I should add that I had a couple previous ruptures with ex-MC that we *did* work through. The difference those times was that he ultimately accepted some sort of responsibility for making a mistake (though it took some time on one occasion) and still showed himself to be accepting of me.