Is anything required of me after I made up past days of fasting without knowing that it was obligatory to make them up due to waswās (obsessive whispering), and is there any beneficial guidance for me and for those who suffer from waswās?

There is no better cure for intrusive thoughts than to ignore them and pay them no mind. Nothing is held against you. You will be rewarded for your patience with this affliction and for striving against your soul to rid yourself of it. And whatever you fasted that was not obligatory for you to fast, Allah will not let its reward be lost; rather, it will be considered a voluntary act.