What causes the painting to get worse and worse?

Bottleneck period, if this is the reason, it is too normal. Everyone who paints will experience it, just like losing weight. To a certain extent, the weight will not drop or even rebound. This is because the body is adjusting, so don't worry about continuing to work hard. The same is true of painting. After a period of time, painting skills will decline, and even things that were painted well before can't be painted well now. It doesn't matter, this is your brain making adjustments.