What is the ruling regarding excessive obsessive thoughts concerning impurities that nullify ablution, and what should be done about the recurring feeling of needing to wash one's hands after touching water used for istinja' (washing after urinating)?

You must turn away from whisperings and ignore them. Water that separates after purifying urine is pure if it separates without being altered by the impurity and the spot has been purified. Washing the impurity on the hand once by pouring water over it is sufficient if the substance of the impurity has been removed.