Martina,
First of all good for you for seeking what you needed. I would just tell her that you are going to try working with a new therapist. That you appreciate all the progress that she has helped you achieve and that you would still like to continue working with her with regard to your meds. I would not go into a lot of detail about why you decided to change therapist, just if necessary say that you just sensed you needed to change things up a bit. Keep your options open so that you can change back if the spirit moves you.

Therapy is about you not the therapist. As long as you don't give her a laundry list of all the things she didn't do for you, there should be no negative feelings about you wanting to explore a different approach or person. She will likely take pride in the fact that she helped you move from being totally out of control to being very proactive in meeting your own needs.

As for your new T. It sounds like he is working out well. I get why he wants you to tell her but I don't get why he thinks it is necessary to get a history. If you have been working well so far, why can't he just get your history from you?