You are not using the proper grip and the handgun is not in alignment with the strong hand. Most shooter do not use proper alignment.
Here is a clip of Todd Jarrett on the fundamentals of the semi-auto handgun grip. Todd Jarrett starts to cover alignment at 0:38. Notice every time he shoots that the entire backstrap is covered by the palm of his strong hand. Also notice that the support hand fills the void on the grip - the support hand is on the firearm and not on the strong hand. When the trigger finger is on the frame just below the slide, the support hand thumb tip and trigger finger tip should be almost straight across from each other. If the handgun is aligned properly with the strong hand and support hand, felt recoil is less and the recoil will not be going into the thumb knuckle.