Smo,
Try it this way,
Keep your head upright and put gun to your shoulder look over the barrel.
If you can see a lot of barrel and not just the tang and front sight , all the better.
Holding it like this, is your eye over the centre of the breech? I fit is, you should be OK for right & left unless barrel is already bent.
Put your left thumb over the top of the barrel. Do this where your forward hand normally holds the gun.
Can you still see the front bead? If so, Good! This should mean that holding the gun in this fashion, you are now giving it some extra elevation.
What I think was happening is that if you are dropping your head to look 'down the sights" so to speak, you were running into the comb, and it wasn't allowing your head to get over to the centre of the barrel quite.
And, with dropping your head, you were "lowering the back sight ". (shooting low) If you keep head upright, and just look Over the barrel rather than Along it, you Will shoot higher.
Many good shotguns come up in this fashion, so that you see the full length of the barrels, not just breech and front sight. In fact All the shotguns I can shoot with, if I look at the barrels at all, they are always in full view all the way from breech to muzzle.
One other thing, shoot with both eyes open!
Sorry, this must sound like you're back at school. It's not meant that way, but rifles and shotguns are very different. See if it helps.