You don't need to leave this therapist. You have been doing really well with him. Right now you are reacting (understandably) to finding out about the diagnosis, and your emotions are all over the place as you work through that (again, understandably). Even if it right now it feels like quitting would be good, you can be assured that once the emotions all settle down you will feel differently about this ... you won't know yet in which way you will feel differently about it, but you can be assured it won't be the same as you feel -now-. I made it a rule that I couldn't make any major decisions until I was done with the emotional backlash from any given thing... because the emotions are but fleeting - they will *always* change. Decisions can be made once the feelings have settled down.
Later, when the emotional storm subsides you will be able to think clearly from a more rational and less emotional place. Then you will have a much better understanding of the right choice to make for you.