I've played chess for almost 13 years now (I'm 23), and I don't know how to improve my game level. I'm rated ~1400 FIDE, and have never been higher. I'm also ~1300 on chess.com after 700 games, and between 1700 and 1900 on their tactics problems. But even with this, I'm still losing games against 1200 ratings.
However, I don't think that I have a bad level. I win regularly against 1600-1700 in official tournaments in South France, and I finished 38th out of 200 in the ~1500 French Championship in 2018.
I think that my biggest problem is that I don't recognize when a good move is playable or if I'm winning on the board.
I never worked on tactics, openings and stuff in my youth. Maybe it's the cause of my problem, but today, I don't improve my level much. I can sometimes win against 1600 but still losing against 1200, and more than that. I saw new players that have begun because of The Queen's Gambit and are already almost better than me. I also saw someone on this forum become 1900 in only 2 years!
So I'm really questioning my chess level at the moment, but also questioning my learning. I would like to be given some very nice tips or advice.