"I missed a whole bunch of fasts over the last decade or more due to illness, menstruation, and pregnancy. Do I have to make up these fasts? It seems too difficult a task for me to accomplish."
This question isn’t answered in the Quran even indirectly. The Quran obligates us to fast, but it doesn't say what happens if you’ve missed fasts, especially if a year has gone by. In the hadith, it is said that Aisha, the Mother Of The Believers (may God be pleased with her) used to make up her missed fasts just before the next Ramadan started. But that doesn’t answer what happens if the next Ramadan comes and the missed fasts haven’t been made up.
Given this lack of clarity, scholars have had to rely on their intellects to arrive at a reasonable answer. Some take a very strict view: not only do you have to fast the days gone by, but you also have a penalty (you must give a certain amount of food grains to the poor, which may amount to two kilograms of flour or $6 or feeding one poor person one meal for each day of the missed fast). Some even said that if you have this outstanding fast from two years ago, then the penalty becomes multiplied. The problem is that it can easily multiply and quadruple until it becomes difficult for the person to fulfil this obligation.
In my humble view, we need to simplify this ruling. God didn't say anything about this. Only human beings have. The result is that people are asking questions on websites about what they should do. This is a frequent question, especially from women. And the answers are not very easy to live by. Some may say, for example, you fast every Monday and Thursday. You fast three days a month, every month and so on. And that can be a burden and require a long-term plan. For a long time, you have to live with the idea that you’re not freed from some past obligation. Psychologically, this can be very difficult for people.
God has said in the Quran, you fast and God desires for you to complete this number of days. And He desires ease for you, not difficulty.
In my opinion, if someone has missed the fasts of the last Ramadan, they should try to compete those fasts before the next Ramadan comes. If the next Ramadan approaches and you didn't finish fasting the days that were missed from the last Ramadan, then you should give fidya for each day of the fast, which amounts to one meal for one per person for each day. And if you don't have the money to pay for the food, then that too is waived, because every obligation in Islam is dependent upon one's ability to carry it out. And since this is a financial obligation and you don't have the financial ability, then you are freed from this obligation. And this is simple. Psychologically it is more pleasing and it's doable. And it means that when the next Ramadan comes, we can approach it with the spirit of happiness and excitement.