Work a Little Harder, Work Another Way
Nevertheless, I've put some work into my game over the past couple of years and have improved quite a bit, at least for an adult. I went from approximately a 'B' player to a modest 'A' player. The improvement was gradual, but not steady, and discouraging plateaus were a common experience.
This may be because the knowledge required to play chess well is of such a range and depth that it's very difficult to pinpoint what you're doing wrong and what you need to learn. Every so often, you fix a hole in your game almost by accident and you take a leap in understanding and rating.
Identifying those holes would require a coach, but there are at least three things you can do that should always have a measurable impact on your play.
1) You can never solve too many puzzles.
Earlier in the year, I noticed I'd done 39 hours of tactics training, and felt proud. Then I thought about it: you need 10,000 hours to get good at something. 39 hours is peanuts! Do better than me, and try to do at least 20 minutes of puzzle solving every day.
Repetition is important (repetition is important). You need to get to the point where if you see a puzzle you've done several times before, you a) recognise the position, b) remember the solution c) are able to solve it within seconds without thinking.
Here's a puzzle every single chess player should know, from the end of Morphy's Opera Box Game:
If you see a complex puzzle you've never encountered before, you need to have the attitude that you won't just give up and look at the solution, but have a real try at it:
When you're solving, make an effort to identify tactics, themes, and patterns, and to link them to previous puzzles you've seen. The more interconnected your knowledge is, the more likely you are to retain it.
2) Try blindfold chess.
I can't play blindfold chess. Certainly not well. But studying it has improved my sighted game. Allow chess to bypass your retinas and launch directly into your visual cortex and you will subconsciously play better.
Start with learning the colours of all the squares on the chessboard. Close your eyes, imagine what colour g7 is, and open them again to check. Something I find useful is that you're not always able to actually 'see' the colours; most of the time you figure it out through logic. E.g. e1 is where the White King starts, so it must be Black. The a8 square must be White as it is at the Black player's right corner ('white on the right'). The c4 square must be White as the King's Bishop goes there in the Italian Game. Etc. After this basic step, there are countless other exercises you can perform to improve your visualisation, many of which are described elsewhere on this site. One idea: try moving a Knight around a chessboard in your head. Start with a square – say, b1 – and figure out how many moves until it reaches a second square – say, g8. You would try to picture the Knight going b1-c3-d5-f6-g8.
3) Build a proper opening repertoire.
This will require some resources, such as databases, books, articles, videos. Pick one first move for White and one Black defence to each White first move. You may have to experiment first to find openings you're comfortable with. Go nuts: google every opening in existence and try to play each one at least once!
When you get serious, make legitimate choices that will stand up over time. 'Dubious' lines like the Dutch or Smith-Morra Gambit are fine as long as you understand it and are comfortable with the positions, 'unsound' lines like the Elephant Gambit are not. Playing an unsound opening is psychologically convenient, as if you lose the game you can blame it on the opening and thus keep your ego intact, but it will not help you improve as a player. At some point you need exposure to the grand-daddies of chess strategy and tactics: The Spanish, Queens Gambit, Sicilian, Nimzo-Indian, etc.
Learn principles, learn some lines (it doesn't have to be very deep), learn traps, learn positional themes, learn typical sacrifices, study famous players who have used it, and look at dozens of GM games. Fall in love with an opening and it will repay you. It's a nice feeling to beat a stronger player because they play into a sideline which you understand better than they do.
So that's it for now. Play around with these suggestions. If you're not seeing improvement, you could try working harder – or just trying something new. Either way, we'll all float on ok ;)