Must I make up the days during which I practiced masturbation in Ramadān last year, given my ignorance at the time that it was among the things that invalidate the fast, and my feeling of guilt then?

Scholars have differed regarding one who unknowingly commits an act that breaks their fast, whether they are required to make up for it or not. The fatwa (definition) adopted by us is that they are not required to make up for it. Accordingly, the preponderant opinion is that you do not have to make up for it, but if you were to make it up as a precaution, that would be good. You must also repent to Allah Almighty for what you used to commit, such as viewing impermissible things and engaging in the bad habit.