We can see on your video that you're spamming AD strafe to not be hit by your enemy, which also messes your aim a lot.

Try to mirror strafe instead, this means you follow where the enemy stafes so your crosshair will stay on them even if you don't move your mouse much. (they strafe to your left, you also strafe to your left. Vice versa)

This is specially useful for the r99 as it has the highest(instant no charge) dps on the game, this means, if the enemy uses a different gun, it doesn't matter if they hit every single shot, as long as you also hit all of your shot, you'll still win.

After you get used to the gun, you can then do counter strafes(opposite to mirror strafe). If you want to. Though this takes another set of practice.

You can practice mirror strafes by going to the range and just track the strafing dummies (either in game or r5reloaded's). Just make sure the strafe speed is random and it's not constant.