How can I improve after 10+ years of chess? 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.
Some suggestions, in addition to doing some tactics regularly (that's number one).

Play at a time-control that is comfortable and not too fast, especially if you are playing blitz.(eg: switch to 5 min if you normally play 3 min); improvement is difficult anyway by playing only blitz games. Slow chess is needed at some point.
Analyze your games and see what kind of mistakes that you are making. You might even be repeating a tactical mistake in an opening and these are things that can be noticed and avoided.
Pick some opening/strategy of your choice and play that every game; over time, you'll be familiar with it and get some confidence.
Learn some basic endgames; knowing at least simple ones like King+pawn vs King will give some confidence; you can build on that and learn Bishop vs 1 pawn ending, R vs pawn.
Play stronger opponents whenever you get a chance (there are settings you can adjust to seek only players above a rating). This will force you to raise your level.