What is the religious ruling regarding zakat for previous years, given that I paid it only on cash without including gold due to ignorance, and I did not know that it must be calculated by the lesser of the two nisabs? Also, I cannot recall the amount of money in those years.

There is no obligation for someone who paid zakat on their money based on the gold nisab, and then realized that the silver nisab or the lower of the two should be used, to re-pay zakat for previous periods.

Banknotes are to be combined with gold, silver, and trade goods to complete the nisab, a principle affirmed by Fiqh academies and councils of senior scholars.

If someone was paying zakat on money without combining it with gold or silver, or did not pay zakat on either of them because neither reached the nisab individually, then there is no obligation upon them concerning what has passed.

The muzakki (person paying zakat) must observe the nisab of gold or silver (whichever is less) and combine banknotes with them from the time they become aware of this obligation.