Getting worse does happen. Over my many years of rhythm gaming I've had quite a few periods where my skill declined. When this happens I do 2 things: I try not to focus on getting better, and just play whatever I feel like for fun; and I try to figure out exactly what skills are getting weaker. When you can figure out what skills are getting weaker, you can focus practice on maps that test those skills (for example, if you find your accuracy is getting worse, you can focus on playing easier songs and working on your accuracy. If your reading is a problem, you can focus on playing different maps with diverse patterns, find out which patterns are problems and focus on playing maps with those specific patterns).