while watching ROTJ again it came to me that a wonderful catalyst for Anakin turning to the dark side is him getting so seriously injured, disfigured, requiring cybernetic modifications and being disabled in that he has to wear a sealed suit. that would mess you up mentally. what could he do? he can longer be intimate with his wife. he's more machine now than man. but there's Palpatine there giving him hope.

so in my mind Anakin wasn't perfectly fine up until the Obi-Wan duel. it was in some other duel or several others that he got that way. he probably wore a prototype Vader suit without the classic Sith style influences. that was added later after he turned.