Is it obligatory to make up the prayers during which the questioner doubted her purity, and is it necessary for her to make up the prayers for which a fatwa was previously issued stating that they did not need to be repeated, knowing that she used to follow the opinion of Ibn Taymiyyah and sometimes the opinion of the majority out of caution?

You should not give in to obsessive thoughts. The matter of purity from menstruation is clear; a woman recognizes it by one of two signs: dryness or the white discharge. If she doubts the attainment of purity, she is not obliged to perform ghusl until she is certain, nor is she obliged to make up the prayers she missed before certainty. However, if she becomes certain of purity, she must hasten to perform ghusl. If she delays it until the prayer time has elapsed, she is obliged to make up that prayer according to the majority of scholars. Her ghusl before the complete cessation of menstruation is not valid, and she would have been required to repeat it after seeing the drops. The view that it is not obligatory to make up prayers for someone who missed a condition or pillar due to ignorance of it is the choice of Shaykh al-Islam Ibn Taymiyyah, and it is a sound madhhab, even though the fatwa we adhere to is the view of the majority due to its precaution.