How can one distinguish between obsessive whispers (waswas) and the feeling of obligation to repeat ritual purification (tahara) or prayer (salat), especially after overcoming obsessive whispers on most days?

Waswas (obsessive whispering/thoughts) is one of the worst ailments, and its cure is to turn away from it and not pay attention to it. So, whenever it is highly probable that doubts are due to waswas, they should be disregarded. Some scholars are of the view that the frequent doubt which a servant is commanded to ignore is that which recurs every day, even if only once. As for doubt concerning an act of worship after completing it, it should not be paid attention to and does not affect the validity of the worship.