Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don’t understand it, but by that time you are so used to it, it doesn’t bother you any more.
– Arnold Sommerfeld
Sounds about right in my case. In high school, I understood enough to get by (as long as you surpass classmates in understanding, that’s all that counts?). In college general chemistry, I was confident about them (my prof told me years later that I could’ve skipped going to class and still been at the top of all the gen chem sections). Then while taking thermodynamics and statistical mechanics in graduate school, the sentiment went, “wait, what? oh, whatever.”