Ive had this experience, sometimes rather pronouncedly where the feeling of exhaustion and depression lasted for days. I found for myself that longer sessions made sub-drop more likely, and I also found that a lot had to do with my expectations and state of mind beforehand. I try now to schedule sessions that are no more than 2 hours, to cultivate a state of calm beforehand and to try to clear my mind as much as possible. I also find that the rapport with the Domme matters a great deal; the more rapport and connection and mutual comprehension, the more elated rather than deflated I feel afterwards.

I also plan a degree of self-care after a session, such as a meal out, and a good nights sleep.