You can outright lose by playing the wrong first 4 moves in the opening.
Even if you played a decent opening, if you misread once, the game is usually over. Your life and death must be impeccable.
The following statements are interchangeable:
- 9x9 is all about sente
- 9x9 is all endgame
I usually say that 9x9 consists of the opening until move 8, middle game until move 16 and the rest is just endgame.
In order to refine your 9x9 skills:
- study a single opening you like the most (per color)
- grind tsumego, properly
- practice counting quickly and accurately - at every move
There is very little strategy to 9x9, most of it is pure reading. Look at the move you want to play instinctively, play out the respective endgame in your mind, count points, if you lose - consider the next move,… if you can’t find a winning move, go with your instinct.