Have you ever learned something truly? That you didn't forget? If the answer is yes, try to think about the environment that helped you do it and emulate it. Unless it requires socializing with people or something that is uneasy to do on your own, it should be doable.

E.g. if you found that a particular type of class in college helped you a lot, say, a laboratory where you had to hand in code weekly, then you could "assign" yourself a task that you'd like to do, just in a laboratory fashion.

Sometimes you might think this is hard for a newbie to do, since you don't know what you don't know, etc. For some things you can look up a relevant course at a "good" university/program and see what its curriculum is, what the reading material is, what kind of tasks get assigned etc.