You will forget anything you don't use on a regular basis, the important thing is to keep practising and using the important things.
Can I remember the threshold for number of seats against fire axes and cabin crew required? No.
Do I understand the limits related to an approach ban? Absolutely.
Just keep revising the important facts, and maybe make a little booklet you can carry around with the important figures in, so you can use them when you're flying if you forget.
With most of the rules it's not about being able to remember everything right away, it's about knowing where to look it up and get a precise answer.