I don't really have any advice, but I do want to say I empathise.

A few weeks ago I abruptly quit therapy after a year of sessions, as my otherwise excellent therapist refused to hug me. Stopping was the right decision, but a week after I quit, I called my therapist to ask for a final session to put some closure to the relationship, which I was already devastated to lose. He refused. I found, and still find, his answer heartbreaking. I'm really confused and grieving for the loss of something that was very dear to me.

I honestly think the best thing to do in these situations is to try and decide what you can take forward with you - and that's what you got from therapy, not the therapist. What did your therapist help you with? What can you do to honour what you learned? What can you do to make sure your grief isn't pointless?

And if the person you need an ending from can't/won't give you one, make your own. No, it won't fix the supervisor's decision, but you can stop giving her hold over your feelings.

I think humans are wired to need ritual. I booked a single session with a random counsellor where I had a 'goodbye to therapy and all that' session. It's not the same, obviously, but it has helped. Perhaps you could do something to mark the end? Plan it out and treat it with the seriousness it deserves.

I'm sorry this happened to you. Loss is hard, no matter the shape it comes in.