Firearm proficiency is a martial art. And like any martial art, you must practice your art or you lose your proficiency.
You also need to be in shape. I see so many students who are physically weak to the point that they can not even keep their firearm pointed at the target and at eye level for a mere magazine of shots. Upper body strength and breath control is essential to your shooting skills.
The cure? Exercise. Build your strength. Build your endurance. Your life depends on it in so many ways.