What should I do about obsessive thoughts regarding impurity, especially after using a towel that I suspect became contaminated by touching an impure staircase? And must I purify everything it touched thereafter, or should I act upon the principle that certainty is not removed by doubt?

What afflicted the questioner is a satanic whisper aiming to corrupt her worship. The cure for this is to completely ignore it and to treat everything she doubts its impurity as pure. The doubt of one afflicted by whispers is inconsequential, and by doing so, she will get rid of the whispers.