Ever check yourself for eye dominance? That's the first thing I do with a new shooter, even before we get to the safety lecture, before we ever put hands on a firearm. If you're cross dominant, that could affect your shooting form in a negative way. To determine your dominant eye, extend both arms out in front of you. Make a circle with your thumbs and fingers. With both eyes OPEN, center up a small object at least ten feet away in the circle. Close one eye, then the other. The one eye open that keeps the object centered in the circle is your dominant eye. It's usually best if your shooting 'handedness' is on the same side as your dominant eye.
Getting a shotgun butt into your shoulder pocket is important in avoiding pain. To find your shoulder pocket, but your support hand (aka 'weak hand') thumb on the middle of front of your collar bone, the extended middle finger of the same hand on the front of the 'ball' of your shoulder joint, and extend the index finger (pointing finger) down to your shoulder halfway between thumb and middle finger. That 'soft spot' is your shoulder pocket. That's where you want the butt of the shotgun to go. Bring your elbow up to shoulder level with the arm flexed, and swing that elbow forward and back slowly while pressing in on the shoulder pocket. Feel the bone on both sides of your index finger? Getting a shotgun butt on bone hurts. Getting the butt out on the muscle of the upper arm on the outside of the shoulder joint hurts too. Getting into the habit of a good consistent gun mount with the butt of the shotgun in the shoulder pocket is important.
Gun fit is important too. IIRC the LOP of the stock on the Pardner pumps is about 13", which is 1" to 1 1/2" or so shorter than many production guns. It could be the stock is still too long. It's usually easier to shoot a shotgun (especially a defensive shotgun) with a too-short stock than a too-long stock.
If recoil is such that it hampers your ability and willingness to shoot the gun regularly, it might be you need to go to a 20 gauge. There's a very nice youth model 20 ga. in the Pardner line if you need a somewhat smaller sized gun, also.
If you're a normal, healthy person, shooting a shotgun shouldn't hurt that much. The advice to start out with lighter recoiling loads is very good. Whatever the problem is, I hope you get it sorted out soon and with no more pain.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.