Here's some advice I've been given by maths professors that I've put into practise. This stuff really works.
-- Make sure you can do it cold, as in make sure you can do things without looking at notes, or looking at wherever you learned it from. For example, say I give you a calculus problem, you should be able to solve it without any outside help, just you, some paper and a pen. No notes, no Google, nothing else. If you can't, then you need to study more and do more problems.
-- Build a routine. Make sure you study whatever it is you want to study every day at the same time and you'll find yourself wanting to do it after a habit is built. And if you miss a session, for whatever reason, you'll feel quite bad about it, and want to try extra hard in the next session because you know you're "behind schedule".
-- Actively recall whatever you've learnt. You can do this by quizzing yourself (make your own problem sets, or do problem sets made by others), and by using flashcards/anki for the things you have trouble memorising. This is one of the best ways to retain info.
-- Don't stress or get angry. You'll just stop the learning process. If you find yourself stressed, or angry, take a break. Remain calm, happy and curious.
You can do it. I believe in you. Start today.