No need to explain or justify it, just be polite and don’t waste their time (as in, cancel future appointments, don’t be a no show). Email template above is perfect. They may request feedback but that’s not compulsory. Ending therapy is normal and happens. Switching therapists is normal and happens all the time! If anything it shows you’ve learnt something from your sessions and you’re advocating for yourself, so be proud of yourself! Wishing you all the best.