Are the prayers I performed before knowing the correct method of ritual purification (ghusl) from menstruation valid, and is repentance sufficient for me, or must I make them up, knowing that they are very numerous?

If one intended to remove ritual impurity (hadath) and covered their entire body with water, the ghusl would be sufficient. However, if you did not cover your entire body, the ghusl is not valid. What is now obligatory for you is to count the prayers you performed in the aforementioned manner and then make them up. This is the view of the majority of scholars. The making up of these prayers should be done according to your capacity, in a way that does not harm your body or your livelihood. If you are unable to ascertain the number of prayers, then you should make up what you are certain of, or what you predominantly believe, will absolve your responsibility. As for the period during which you abandoned prayer, repentance is obligatory for that.