Must I make up for the days when I doubted whether water had entered my stomach during fasting, knowing that I do not remember if I had broken the intention of fasting on one of those occasions or not?

Fasting is not invalidated by doubting whether water reached the throat or by intending to break the fast. If one is certain that water reached the throat unintentionally during ghusl (ritual bath), the predominant opinion is that fasting is not invalidated. Making up the fast (qada') is only obligatory when one is certain of having done something that necessitates it.