What should a woman do who missed days of Ramadan for years due to menstruation and did not make them up, and now feels regret for not remembering the number of years and days she missed?

You have committed the sin of negligence in making up for missed Ramadan fasts. You must hasten to repent to Allah and make up for all the days you missed from past Ramadans, until you are reasonably certain that your obligation has been fulfilled.

You are also required to pay the expiation for delay (kaffarat al-ta'khir) if you were aware of the prohibition of delaying the makeup fasts. This expiation is approximately 750 grams of the staple food of your locality for each day, and it should be given to the poor. However, if the delay was due to ignorance or forgetfulness, then no expiation is required of you; only the makeup fasts are obligatory.