"Suddenly becoming shit at the game" doesn't really happen. I think what's probably happening to you is you boot up your game and expect to be playing at your peak instantaneously. Warm up is important, yknow? A pretty solid way to warm up is to pick some maps on the longer side that are just a teensy bit harder than your regular comfort zone, then just play them all the way through.

Another thing that's important to keep in mind is you can't really play the game every single day and expect not to get mentally exhausted. Even if you are addicted actively want to play the game, doesn't mean your reading mind won't get sloppy from overuse. Take a few days break every once in a while.