What's some valuable things you've learned this year?

I'll start.
Always ask myself, if I can elaborate on any given statement, otherwise just keep shut.

Always ask yourself before writing any code, if it's gonna be easy to modify later on, if not, take a step back and evaluate the bigger picture.

Don't bother with writing things clean in the first go.
It'll be much easier to refactor later on and take less total time.