My surgeon used a polysyllabic word yesterday that I figured from context meant something like:

dwelling [perhaps on negative outcomes]
or thinking about something too much--perhaps obsessively
picking something to pieces mentally

His polysyllabic word sounded something like preseverate--but maybe only in a couple of syllables.

The context was I described to him how very worried I am that I'll ever fully recover my musical performance capacity since I have a plate in my arm--I notice every single nuance of what I cannot do. And he told me several tines to stop ____________________________ing.

I can't find his unusual word. Can you figure it out?