Does merely doubting the validity of the fast, or intending to repeat the fast, invalidate it?

We advise you to turn away from whisperings (waswas) and doubts, for that is the optimal treatment for obsessive whisperings. Your fast is valid, and you are not required to make up any days as long as you are not certain that you committed an act that invalidates it. The default is the validity of an act of worship until something definitively spoils it.