I am sorry you are feeling so much grief at the impending loss of your therapist. This is the nurturer that you have had for years. It is totally understandable that you would feel grief.

What should you do now is continue what is working. If the meds keep you fairly stable keep on with them. You might talk to the psychiatrist about getting a mild prescription for occasional anxiety or depression due to the change of therapists.

Your therapist sounds wonderful.
1) Ask them to evaluate where you were in therapy when you started, what the goals of therapy were, what goals have been met or exceeded and what goals are still being worked on.

2) Ask if email contact is possible after the T retires. If that is not abused by the patient as a constant crutch it might help bridge the gap.

3) How can you deal with the grief and loss you feel?

4) Have I stabilized enough to live without falling apart over this change?

5) What can I do to make this transition easier?

6) What can I carry with me as a remembrance of the sacred trust that we shared for a number of years?

These are the questions I would ask. Perhaps they will help you form your own questions in your own words.