With one of my handguns, my first .45, I was consistently shooting several inches low at all ranges. At 50 yards I was gut-shooting the silhouette with a head hold. I brought gun back to the store and complained, at which point the clerk took me and the gun back to the range and proceeded to fire a 5 shoot group with all shots touching at 15 yards. He had me shoot a mag and pronounced that I was flinching. It wasn't horrible, I wasn't scattering shots all over the map, but I was tensing up before every shot.
I said that that couldn't be it because I was shooting from a rest some of the time.
"So, if you can't lever the muzzle down, you can lever the rear UP with the same tensing motion. That'll still throw your shots low."
I stood there with my mouth open for a minute, because that's actually pretty simple physics. When you exert a force it's got to go somewhere. By using a rest, all you are doing is shifting the most solid fulcrum of the lever forward to where the gun meets the rest, before the fulcrum was your hands. So with the same force, it can't pull the muzzle down, so it shifts your hands slightly UP. low shots.
Before changing anything, get several boxes of ammo from the same lot, 230 grain CCO Blazers will work perfectly. load 1 round in the magazine, load and shoot 1 round. Concentrate on providing a firm, consistent grip throughout the firing cycle. Concentrate on keeping your trigger stroke smooth and consistent, and ONLY in your trigger finger. It is very easy to end up squeezing the rest of the fingers of your hand in sympathy with the trigger finger. Concentrate on isolating that finger.
Dry Fire! Get some snap caps and dry fire. concentrate on a SMALL spot on the wall, perch that on top of the front post and concentrate on squeezing off a "shot" without disrupting that picture.
Another thing that causes pulled shots is attempting to wrap your finger around the front of the trigger guard, that levers the muzzle down. Wrap the non-shooting hand over the shooting hand and stack your thumbs, shooting thumb above the non-shooting.
Also, concentrate on squeezing the grip ONLY from forward to rear. This will take any twisting action out of your grip and help control recoil. Spend a lot of time just getting comfortable with the gun without shooting it. Your Significant other will accuse you of playing with it, but you are just figuring it out.
Once you think you've got that all figured out, head to the range and load 2 or three rounds in the mag ONLY. With only a couple of rounds to shoot, recoil anticipation will be less of an issue.
I still practice these drills regularly to keep my technique clean.