HOW YOU DEAL WITH GETTING WORSE AT SOMETHING?

I know I'm a bit late to this party, but it's an interesting problem that I'm faced with very directly, and I disagree with almost everyone before so maybe my perspective will be useful.

Honestly, getting worse happens. Often it's just in your head, but sometimes not, and it's kinda important to introspect until you're able to tell in which category you fall.

I bet your favorite book by your favorite author isn't their very last one.
There's a lot of subjectivity involved, but there are many objective cases as well - not even talking about things like physical condition, it's known in math that before forty is when you're most likely to get things done.

Mostly, it means you're becoming someone else. With different priorities and possibilities. Sometimes it's worth fighting against, sometimes not, depending on what you gain from this change. And that's not something we can decide for you.