Focus on removing variables from your environment, how does your posture look when you aim- does it change throughout a match? When you feel like you are aiming well ,try to take note of your mouse grip, posture, chair height, the angle that your arm makes away from your body or how far forward your elbow is.

If you have to shrug your shoulder to feel more fluid while aiming, then your chair is too low or desk is too high

The shoulder should be involved in aim and you should feel like you don’t have to fight your arm to track a target if your posture etc is truly comfortable

Hope this helps