I wholly agree with those who recommend dry firing. Try it this way:
Put up an aiming point, such as a 1" black circle in the middle of a 6" square of file folder cardboard at a distance of 20 feet.
Stand with feet shoulder width apart at 90 degrees to the line of fire. Rotate your heels back 3/4-1" to slightly open the stance. Breath a little deeply three times, then mount the rifle by using trigger hand on butt to place the butt in the same place on the shoulder each time. Keep breathing. Adjust front hand by moving to front or rear so rifle is supported without muscle tension. Place trigger hand so trigger finger lands naturally without forcing. Place head on stock by lifting head, setting jaw on stock, then sliding down to get alignment of eye behind sights. A little cant- tilt of the rifle- won't hurt with a muzzleloader as the sight line is pretty close to the bore line. Get the sights in front of your eye with your head fairly erect.
Now align the sights. Look at the black target across the aligned sights, then draw your focus back to the front sight. Keeping your sights aligned and your focus on the front sight, aim and smooooooothly press through the trigger. After the trigger breaks maintain alignment, focus and aim for a mental count of three.
Try to do this three times per week. Do at least ten reps, twenty is better. Do the same drill when firing with powder, patch and ball. Use a mantra- "Alignment-Focus-Aim-Smooth-Follow" to get your firing sequence completed in 5-8 seconds after mounting the rifle. Holds are not like wine- they do not improve with age. Visual acuity fades after 10 seconds even in 16 year old eyes.
White Fox, in the People's republic of Boulder