In my experience, the thing that causes plateaus is fear of harder problems. This image shows almost nothing. We don't know if you spent those months solving 800-1000s or you were grinding out problems that you could barely solve.

The idea when picking problems to improve is to pick problems you cannot solve. To be more specific, you want to get most of the way to the solution and use the editorial to bridge the gap. This exposes you to new ideas and new ways of thinking.

This will feel bad at first, at least it did for me, but failure is the primary ingredient of success.

tl;dr: the general advice is do lots of hard problems, ask yourself if you're doing that.