Your brain isn’t a hard drive that fills up. Learning one thing usually makes the next thing easier to hold.

Memory does not work by filling up, which is both a relief and useful to know. Knowledge forms connections, so each thing you understand gives the next thing somewhere to attach, and that is why people deep in a field absorb new material so fast. The fear of learning something useless and crowding out something important has no mechanism behind it. Unused things fade on their own.

Learn the thing you have been putting off on the grounds that it is not important enough. Notice how much of what you already know it hooks into.