Perfectly normal reaction when you are beginning to shoot a flintlock. After all, there is an explosion going on right in front of your eyes. So, let's work through it.
First, wear glasses to protect your eyes and your perception of what's going on.
Now, grip the wrist of the stock firmly, but not tightly with your left hand, being a lefty like me, and snug it back into your shoulder. Then get a good, solid weld between your cheek and the stock and relax the rest of your body. Doing this will make your whole body absorb recoil gently instead of sharply in the face and shoulder.
Find your natural point of aim, adjust your feet, knees or seat as necessary so that you don't have to wrestle the muzzle toward the target, breathe in, let half out and hold it, burn your concentration on your front sight (you'll still see the target) and the first time the front sight moves to the target, squeeze the trigger and keep focusing on that front sight for the count of three. I promise, this will help.