What must one who used nasal drops (Atrovent) for twenty years while fasting, believing that they do not break the fast, then came to know that they do, do?

Nose drops do not break the fast unless they reach the throat. If they reach it and the patient tastes them, and cannot do without them during the daytime in Ramadan (definition), and there is no suitable alternative for them, and his illness is not expected to be cured, then he must feed a poor person for each day. As for fasting the past days while using the drops out of ignorance, there is nothing wrong with it, and it is hoped that Allah will accept it, especially since the issue is a matter of scholarly disagreement. The correct view is that whoever commits an act that breaks the fast out of ignorance is not liable for anything.