Undoubtedly, excessive and constant doubt which makes a person obsessive is the temptation of Shaitan who wants God's servants to become tired and lethargic towards worshipping God and finally despair of Allah's mercy. Therefore, it is necessary on every believer to fight Shaitan and not to pay heed to his temptations so that he who is man's sworn enemy should despair and get ashamed.[1]

Kathir al-Shak is a person who doubts quite often. One such person should ignore his doubts, whether his doubt relates to the number of the Rak’ats of prayers or to parts of prayers or the conditions of prayers. If a person doubts three times in one prayers or doubts once in every three prayers consecutively (e.g. morning, Zuhr and Asr prayers), he is regarded as Kathir al-Shak. And if his doubts are not due to anger, fear or anxiety, he should not pay heed to them.[2] The criterion through which one can consider a person or himself as Kathir al-Shak is the Urf (common view).[3]

As for the second issue, it should be said that the sacred Shari'ah of Islam considers a thing to be impure only when one is certain that it is najis[4] but if he doubts whether it is pure or impure, that thing is treated as pure.[5]

Read answers 808 and 6820 in regard to this issue.