I have gone back to 2 former T's. One I left when I moved away from home. We left on very good terms and I had enjoyed working with her. When I returned it wasn't the same. We had both grown and changed in the 2+ years that had passed and we no longer meshed the way we once had.
I stopped seeing my current T for a year and have returned to her after not being able to find anyone who can help me AND takes my insurance. Re-entry is a B*. It was more difficult for me to re-enter with her and stabilize than it has ever been with a new T. It also has taken longer to rebuild that trust even though I have always trusted her a lot. But... worth it.
just my experiences.