What must I do regarding the missed days of Ramadan that I did not fast due to exhaustion or without a valid excuse, after I learned that fasting them is obligatory?

Breaking the fast during days of qada' (making up missed fasts) without a valid excuse is forbidden. You must make up the days you broke fast, as they are a debt upon you. If you do not know the exact number, then strive to ascertain them and make up the number of days that you believe will absolve you.

In addition to making up the fasts, you must feed one poor person for each day you delayed making it up, according to the majority of scholars, if the delay was in making up the days of Ramadan. However, if the question is about making up for breaking a qada' fast, then you are not required to make up the days you broke fast during your qada' period. Rather, you are only required to make up the original Ramadan days, and there is no fidyah (expiatory feeding) upon you for breaking a qada' fast.