Perhaps you could say you used to work in healthcare, but stopped for health related reasons. You could say you got sick and would rather not discuss in detail if they continue to ask questions. Or you could say after years of working such a demanding job your mind and body needed a break or something. I think most people would respect that, since many people don't want to go into detail about their health with people they don't know too well. They don't have to know it was mental health specifically or all the details. You could even just leave it at you stopped/retired early for personal reasons.