What should I do about the days I missed fasting in previous Ramadans, given that I don't remember the exact number and am unable to feed sixty poor people for each day?

All missed fasting days from past Ramadans, when fasting was obligatory, must be made up. If the exact number of these days is unknown, then a number that is most likely to clear one's conscience must be made up. It is permissible to make up these days intermittently. Mere doubt about having missed fasting days is not to be considered.