When we are learning a subject over years there are stages you cross after which if you look back you would be like "wow! I can't believe this used to be so difficult one year ago". Just saw this talk about that in mathematics and teaching mathematics. I have felt it in every subject.

Part 1: https://youtu.be/zHU1xH6Ogs4

Part 2: https://youtu.be/4HFyWC-YtIk