How can I become a stronger chess player?

I play 1-3 games of 15+10 rapid chess every day. After each game, I review it and solve puzzles for over an hour. I watch famous GM speedrun videos or annotated game videos for over an hour. I attend an offline chess club once a week.

I've been following this routine for one month. My rapid rating, which was in the 800s, has now climbed to 1200. My game accuracy has increased from 60% to over 80%, and blunders have noticeably decreased. Mistakes in the opening phase have almost disappeared. I'm finding tactical opportunities more often.

Still, I'm not satisfied with my games and feel anxious even in winning positions. How can I become a stronger chess player?

You should work on opening variations and endgames. Solving tactical puzzles and doing chess strategies would also help too. Also, analyse your games afterwards to learn from your mistakes.

Thanks for the reply!

When you say to study opening variations, do you mean the variations of the openings I primarily use?

For endgame books, would "Silman's Endgame Course" and "100 Endgames You Must Know" be good choices?

I tried reading books on pawn structure, positional play, and strategy, but I gave up because I couldn't understand them well. What rating level should I aim for before trying to read them again?

No problem.

Yeah, you should focus on lines with the openings you normally play. Because then you would be studying, and learning ideas, and patterns that would help you. However, this is just my opinion, and other people might say otherwise.

I haven't heard of those books before and tbh I don't read chess books. But if you think those chess books are helping you and you are improving, then go for it.
Maybe wait until you reach a good rating e.g. 1500-2000 ish because by then you will have good experience with games and lines and stuff. 

Otherwise maybe look at youtube videos online? 
There's this guy called "Hanging Pawns" on YouTube, who is good.

Do not make blunder

Is it a good idea to study the opening moves and ideas of an opponent's opening?

We are the same when I am playing chess before I discovered that playing random opening will spice things up... That is why I made a spreadsheet that randomize my first 2 moves... After that, I began to enjoy playing it again...

Random Move Generator

Random Move Generator

You're on a good way. I would not waste time on opening study though. Reading a good book on strategy would be way more useful. Silmans endgame book is a great read too.