What must one do to escape the recurring cycle of doubts about the validity of the fasts of days already spent?

The cure for obsessive whispers (waswas) is to ignore them and pay them no mind. Doubting an act of worship after completing it does not affect its validity. If a person consumes something that breaks the fast, being unaware that it is a 'muftir' (fast-breaker), then, according to the preponderant opinion, this does not affect the validity of their fast. It is permissible for someone afflicted with waswas to seek leniency and adopt the easiest of opinions to alleviate hardship. The summary is to disregard doubts and not make up for anything that one doubts the necessity of making up.