There's nothing I would say to my younger self that could easily be said in one or two sentences.

But there are two things I'd like to show my younger self.

The first thing is how much a test suite can speed up development. I knew, then, that tests are good, but was just too lazy to write them. Now I really appreciate tests, and how much they can help you in finding bugs if run every so often.

The second thing is how easy some things are that looked very complicated back then. I always thought that writing a parser or (unthinkable) a simple compiler was horribly complicated - and lots of other things.

Some of this changed while reading other people's code, some by thinking on my own, and the compiler and parser thing by reading Let's build a Compiler

Both of these points are nothing that my younger self believed if being told in one sentence, so I'd have to really show it.