Must I complete the fast for the day on which I masturbated on a Ramadan morning, then make it up later, or should I act as one who has broken their fast?

Intentionally breaking the fast in Ramadan without a valid excuse is one of the major sins. Repentance is obligatory for this, as well as for the bad habits that lead to it. One who intentionally breaks the fast must refrain from eating and drinking for the remainder of the day out of respect for the sanctity of the time, in addition to being obliged to make up that day after Ramadan.