Is it obligatory to make up for the days one did not fast since puberty due to diabetes, or is fasting desirable (mustahabb) and not obligatory? And is fasting currently difficult given the improved regulation of blood sugar?

If the doctor determines that there is a risk in fasting, or if fasting proves to be difficult through experience, then one must break the fast and resort to feeding the poor (fidya). However, if the doctor confirms that there there is no risk, provided blood sugar is controlled, and fasting is not difficult, then making up the fast (qada') is obligatory. The determination of hardship or harm should be referred to a trustworthy Muslim doctor. A diabetic person who is able to fast without hardship or harm is obligated to fast and make up any missed fasts (qada'); they are not permitted to feed the poor instead (fidya). As for one who finds fasting difficult or fears harm, they should break their fast and resort to feeding the poor (fidya) without needing to make up the fast (qada').