Delaying making up missed fasts is a sin, so you have to: 1- repent to Allah, 2- hasten to fast the number of days you think you missed, and 3- feed one poor person for each day if you can afford to do that.

Whoever does not fast some days of Ramadan has to make up those days before the next Ramadan comes. For more, please see the detailed answer.

If a Muslim misses fasting for many years, he/she has to ask Allah for forgiveness and repent to Him, and has to fast the days that he/she missed and feed one poor person for each day. If he/she is poor and cannot feed others, then fasting and repentance is sufficient and he/she does not have to feed others.

The scholars agreed that the menstruating woman should not fast, and that her fast is not valid if she does fast, and that she has to make up the days that she did not fast in Ramadan because of her period. What you have to do is to make up for those days, and repent to Allah for your failing to seek knowledge which led you to commit this prohibited action.