It hasn’t been nearly so long for me, but I’ve managed to hold on to much of the math I’ve learned by re-proving theorems I’d learned and solving problems in my head during downtime, while doing household chores, and while falling asleep. Another benefit is that I am almost never bored since there’s always more math to do.

Just block off regular times to do the math you want to remember a few times per week at least. Maybe even 30+ minutes per day.

Solve problems. Prove things. Etc.