What about just what you said in your post: Tell her that you've decided to separate your therapy from your medication visits, so you will be seeing her for just the medication from now on. Doing this doesn't mean you don't appreciate what she's done to help you, just that you have decided to do things differently.

Some therapists do like to get a treatment history from the previous therapist, but it is your decision to allow that; in fact, you will have to sign a written consent form to allow that. It is up to you.