When you learn something you can't unlearn it.

>>"then unfortunately is replaced by other things"

I don't think that is how it works!

To answer your question:

The best way to use what you have learned is to share it or to connect it to something your are interested in.

I highly doubt high-school Physics would inspire one to become an astronaut, but it some how helps to see the things in a better way.

Always share what you learn and keep learning. At some point your subconscious works things out for you.