When I was serious about Muay Thai I was training about 10 hours a week and constantly injured. There was always something sore, something healing, and some horrifying bruise somewhere. It became obvious that it was time to "retire" from fighting - even though the fights were the lowest level amateur stuff set up between local gyms - when I started having to go to urgent care after every match. It got into sort of a downward spiral of "Get fit, have a fight, get hurt, lose fitness, rebuild to a slightly lower level, fight, get hurt, lose fitness, rebuild to a slightly lower level..."
Eventually common sense won out and I limited myself to partner training and light sparring three times a week. I'd still be doing it if it weren't for Covid, but all the broken ribs still ache when the weather changes, and most of my toes don't line up with each other any more, and...