Does swallowing water unintentionally during ablution, or swallowing phlegm and having doubt about whether it reached the throat, break the fast? What are the implications of forgetting the type of fast being made up (Qada' or Sunnah) and fasting it with the intention of Qada' only? And how can one get rid of obsessive thoughts about fasting?

The remedy for obsessive thoughts regarding fasting and other matters is to ignore them and pay no attention to them. If one doubts whether something has reached the interior of the body (jauf), they should disregard the doubt and act upon the original presumption, which is that nothing has reached it. Water reaching the throat unintentionally does not invalidate the fast. Swallowing phlegm does not invalidate the fast according to many scholars, and someone prone to obsessive thoughts may adopt this view. Doubt concerning the invalidity of make-up fast days should be ignored; the original presumption is that these fasts are valid, and doubt about an act of worship after completing it has no effect.