What do you do about not remembering the number of days you missed in Ramadan due to your menstrual period, with many years having passed since then?

You must do three things: repent to Allah and regret the delay, hasten to fast the days you think you missed, and feed one poor person for each day if you are able, with half a sa' of the staple food of the country (one and a half kilograms). If you are poor and unable to feed, then there is nothing upon you except fasting and repentance.