ANSWER:

Yes, you should pay the zakat for the past 2 years.

From now on, every year before your zakat time, ask her if she can pay it back or not and if she declines, you don’t need to pay zakat for it as she is unable to. 

QUESTION: Two years ago I lent my mom some money(which is above Nisab amount), with the idea that she would pay it back when she was able to. But I forgot that when paying zakat over the last two years that she had that money and didn’t ask if she had it before paying my zakat, should I pay zakat for those two years automatically? And should I ask her before I pay my zakat in the future?