How to improve your rapid

Solve puzzles regularily to train your tactical eye, board awareness, etc. Also learn how to "sense" when tactics could be on the board in an actual game (unprotected or single protected pieces, king safety).

Solve a couple puzzles before playing a game to get your brain into "chess mode".

Analyze your games afterwards to identify your errors and problems.

Watch GM Naroditskys speedruns on YouTube, they are highly instructive.

Get master games, ideally with master commentary, and try to understand them.

Up to about rating 1000, only play slow time controls so you have enough time to really think what move to make.

Blunder check before doing a move; specifically at your level, games are decided through who does less blunders (even at my current level thats still the case and one guy here even told me to get to 2000 all you need to manage is not to fall for or miss any two moves tactics).

Up to around rating 1000, you dont need to learn openings, though knowing the first few moves wont be bad either. Until then, follow general opening principles and play open games, even if you think you are more of a positional player.

Dont trade pieces unless you get a benefit from that trade; because if you dont, then usually the opponent gets a benefit ("To Take Is Mistake").

Manage your time correctly. Find the times when you need to halt and spent time to find a plan or a tactic. Never have far less time than your opponent.

Dont be a total pushover in the endgame. Get a good endgame book, especially if you play with time increment. But even without I already had endgames and was amazed how some players are completely clueless in that area and offer no resistance at all.

Learn and apply the most important principles of chess. - (core of my teaching)
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.