I have played 12 years of guitar. (most of it self learning)
know some finger style.
sometimes, until a certain level. I feel like cant improve or sometimes getting worst.
any way to improve?

Same problem, currently i'm trying to absorb this heheh

get a good teacher
https://www.youtube.com/watch?v=5j44PmN-ChA

go to classes.

If its one thing I've seen over the years, regardless of how far you've gone, there's ALWAYS a next level, whether you're a fledgling noob or Joe Satriani. Guitar playing can be as broad and deep as we want it to be. The musical theory aspect of it, the mechanical aspect of it, the techniques, the compositional aspects of it......there's just so much to cover.

PRACTISE...PRACTISE...PRACTISE...PRACTISE...PRACTISE...

focus on the part that you want to improve, begin slowly then gradually increase

Try learning to cover a new song that you truly enjoy every month and move to the next one. I find this therapeutic and useful to improve and stay interested in learning

Learn music theory if you haven't.
Get a good teacher.
Compose music.

practice is half the story.

I used to tell my students or my bandmates
"PERFECT practice makes perfect"