What should I do to estimate the number of days of missed fasting that I do not remember, and how can I make up for the missed fasts given my semi-chronic illness whose severity varies?

One must clear their conscience by making up for the missed days based on what is most likely to be true. If there is an illness from which recovery is expected, one must wait until recovery and then make up the fasts, as Allah Almighty says: "So whoever among you is ill or on a journey [during them] - then [he must make up] an equal number of other days." However, if the illness is one from which recovery is not expected, then fasting is not obligatory; rather, expiation (fidyah) is obligatory by feeding a poor person for each day, as Allah Almighty says: "And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person." The amount of feeding is one mudd of food (approximately 750 grams) for each day. One can feed thirty poor people at once, or one poor person over thirty days.