I have the same problem
I reached 900 ELO, what should I learn now?

why don't you try new openings like the vienna gambit or try to play with more time to calculate your moves?

maybe you play more games, I got from 400 to 1500 in like a year just by playing games. I only played around 6 games per day on average, didn't play everyday. And after reaching 1500 I drifted out of chess for 1 year, fell back to 1480, began to learn an opening. Once I had started to memorise lines, I (quite obviously) had a strong start in all of my games, which helped me shoot up to 1800. I know that's not quite what you asked, but it is what helped me get above 900 and much higher.

Learn exactly how to think in the opening, middlegame and endgame — this is what I teach.
Always blunder-check your moves.
Solve tactics in the right way.
Analyze your games.
Study games of strong players.
Learn how to be more psychologically resilient.
Work on your time management skills.
Get a coach if you can.

I remember the times when I was 900 (back when blitz was my competitive time control). Good times. Played the Bird.
Anyhow, crossing 1000 can be tough. Don’t worry too much about your openings- if I were a coach I outdone tell all of my u1000s to play opening systems. Pirc & Jobava London can be good played right. Only after getting above 1000 is when we might start paying more attention to the opening.
As for middlegames, keep on drilling in those tactics. If you need more puzzles, check out aimchess.com, they have tons of resources and it can connect to your chesscom account! They also have some lessons over there too, but chesscom lessons are better and they’re daily now, so I’d stick with those.
And lastly endgames, really u1000 to be honest should focus mostly on tactics and something will occur where taking advantage is possible, and you shouldn’t reach too many endgames. Just know your basic mates (knight + bishop DOES NOT COUNT, I don’t even know the trick. Just K+Q, K+R, two queens/two rooks/Q+R). After crossing 1000 fundamentals like opposition, zugzwang, and triangulation can be taught.
I recently reached 900 ELO, but noticed that my opponents are starting to get tougher and tougher, and while I am improving, I feel there is still a lot of work to be done. Can someone help?