yes it is so so hard isn't it. I think that is mother nature's way (some biological thing in our minds) to let us know we still have work to do on ourselves and with this therapist. Unfinished business so to speak.
As to how long it takes, it does vary, I would think as long as you are seeing him, it would be hard not to have these thoughts. Maybe try working them through again and again with him until you are clear about what you need from him, and how he understands your feelings and can help you with them.I don't think there's anything wrong with bringing it up as often as you need to discuss it.
I think sometimes we have to dwell on something until we are well and truly sick of thinking about it! (remembering my teenage crushes on Donny Osmond. ha ha ha. ) I know its not the same. I just remember thinking, Iwill NEVER get over him. LOL!!!!