Sometimes after studying or practicing people get a little worse. The new ideas or skills haven't been properly assimilated into your play yet.
But after this brief period, you will eventually start playing better than you were before.
As for being the same level "a long time" well it just depends. Sometimes it's natural. Keep playing and practicing and you'll overcome it.
Other times, if you've been the same for years, then you probably have bad habits and you'll have to make changes.
For a long time I am at the same level. Now I am getting a feeling that I am worse than before.
I am playing but it seems it is not enough. I am training tactics and trying to see my mistakes but it does not work. Am I doing something wrong or have I reached my maximum level?
There is probably some aspect that I don't understand.
Why am I getting even worse than before?