Must the questioner make up for the prayers she performed in previous years, mistakenly believing she had purified herself from menstruation, only to later discover that her purification was not valid?

Regarding making up prayers performed without proper purification, Sheikh al-Islam Ibn Taymiyyah is of the view that you are not obligated to make them up due to your ignorance of the ruling. You may adopt this opinion to alleviate hardship. However, if you wish to be cautious and follow the opinion of the majority by making up the prayers, you should know that one of the conditions for the validity of the ritual bath (ghusl) after menstruation is the intention to remove the ritual impurity. If you did not have this intention during your ghusl, then your prayers are invalid, and you must make them up. To determine the number of prayers, ascertain and make up what brings you certainty or a strong presumption of being absolved of responsibility. The method of making up prayers is explained in Fatwa No. 72777. If the matter becomes difficult for you, you may adopt the opinion of those who do not consider making up the prayers obligatory.