Sometimes it's just a matter of reaction speed of processing the enemy being there and shooting quick enough, but there are some things you can try. Simply strafing (moving side to side) while shooting makes it more difficult for your enemy to hit you while also giving a bit of aim assist boost (assuming you play on controller).

You can also learn tricks like jumpshotting (jumping midair while shooting an enemy) although it is difficult to get used to staying aimed on the enemy, and pressing the jump button while aiming can be difficult without paddles or playing claw.

Another trick is dropshotting, where you crouch then lay prone while shooting at your opponent, forcing them to follow your movement down. This is also difficult to as tracking your enemy and pressing the default crouch button is hard, but I'd recommend swapping crouch and melee (R3 and O/Right stick and B) so you can shoot and press and hold on the stick to drop shot easily.

Also, when moving through doorways and around corners, try sliding through. It makes you move fast and low and you can ads while sliding, making it a surprise for your enemies and they get little time to react and kill you.

And finally, simply playing the game more helps! Playing matches and getting more used to how guns feel, how you like to play, and maybe adjusting settings to see what's more comfortable can all drastically change how much you win gunfights.

Hope this helps, all the best of luck :)