The live round - snap cap game is a good way to find out how bad a flinch is. The only way to get over it is to mentally make youeself not move.

You just gotta let the gun kick you. It isn't gonna hurt any more than when you are flinching. It's not like the rifle kicks more because you jerk at the shot. Actually the oposite is true. If you allow your shoulder to absorbe the recoil and move with the rifle and don't tense up the felt recoil will be less. Just like someone punching you in the arm. When you tense up it hurts more than when you roll with the punch.