For my own part, it was all about finding the right book. I enjoyed my degree in pure maths, and after finishing I really missed studying so I tried out dozens of books. Eventually, I hit upon Calculus by Spivak. This book fundamentally changed me and I solved every problem in it fastidiously - it was just so well made, funny in parts, it had cliffhangers, and the problems were just amazing, linking together over chapters like characters in a novel. For the first time, I was solving problems just for fun and I was improving much faster than I did during my degree. I wrote all my solutions in LaTeX so I had an organised body of work to look back on and watch grow, adding to the satisfaction (I lose paper; pdf is better for me).
So my advice: find a great book, make notes and expand on what the author is saying with diagrams or additional lines of working, focus on solving problems, do it out of love.
So my advice: find a great book, make notes and expand on what the author is saying with diagrams or additional lines of working, focus on solving problems, do it out of love.