Fasting in Ramadan is one of the pillars of Islam and an obligation upon every adult Muslim who is neither sick nor on a journey. Breaking one’s fasting in the daytime of Ramadan without any excuse is a grave sin that cannot be expiated easily. Prophet Muhammad (peace and blessings be upon him) said, “Whoever broke even one fast of Ramadan without an excuse or sickness, even if he fasted the whole life after that, it will not compensate what he missed” (At-Tirmidhi).
Dr. Sano Koutoub Moustapha, deputy chairman of the International Fiqh Academy and professor of fiqh and its principles at the International Islamic University, Malaysia, states that: if a Muslim missed years of fastings of Ramadan, I advise to repent to Allah and promise that you will never ever miss again any fasting in the rest of your life. You shall equally continue repenting and seeking His forgiveness and mercy.

However, it might not be possible or practical to make up for all these years since you are not sure about their exact number. Therefore, you must feed as much as you can a considerable number of needy people. At the same time, you must try your best to fast as many days as you can in order to compensate for those missed fastings.
May Allah forgive and accept our repentance to Him.