I wrote this a couple weeks ago for someone and it seems to apply to you, too, so here it is again. If some of it doesn't apply to you and your gun, ignore it.
Your shots hitting low is due to you anticipating the shot and jerking the gun a bit right before it fires.There are ways to train yourself out of this "flinching."
1) Dry-Firing. No, it will not hurt your firing pin or any other part of your gun. Tack a small dot on the wall about 12 feet away. Take all ammo out of the room and check your gun to make sure it's empty. Now check it again.
Once you are triply sure that it's empty, position yourself and aim at the dot. Concentrate wholly on the FRONT SIGHT. Naturally, the dot on the wall will be blurred when you're focusing on the front sight, but you'll still be able to see it. Line up the three dots of your rear and front sights, keeping the dot on the wall above the front sight.
Still staring at the front sight and holding as steady as you can, SLOWLY press the trigger until your hear the "click." Did your front sight dip when the trigger broke? If not, that's good! That is what you want to replicate 100% at the range. Rack the slide enough to reset the trigger and repeat 20X, being very deliberate each and every time.
2) FRONT SIGHT!!!! This was mentioned above but bears repeating. It's one of those "holy ****" things that will improve anyone's handgun accuracy. Focus 100% on the front sight, NOT the target. Let the target blur in the distance and let the rear sights blur in the foreground; you want ALL of your attention and focus on that front sight dot.
3) Trigger control. If you squeeze the trigger too quickly you'll probably pull your sights off target. A slow press of the trigger straight back toward your face will ensure that the sights remain on target.
Also, by pressing the trigger slowly, you will not know exactly when the trigger will break (go "click") and that's a good thing. If you know when to expect the "click" then you'll also know when to expect the "boom" when you're live-firing and consequently your subconscious mind will know when to flinch. You want to be surprised by the click (or boom) every time.
Once you can do all this during dry-firing sessions, you are ready for real ammo at the range. One great way to check yourself for flinching at the range is to purchase "snap caps" which are just dummy rounds that can be loaded into your magazine and will feed and chamber just like a real round.
Have a friend randomly load up your mag with, say, 8 live rounds and 4 snap caps. Your back should be turned away so you won't have any idea, at any given time during the shooting session, whether you're squeezing the trigger on a live round or a dummy round. Only when you hear the "click" will know it was a dummy round.
The thing to look for (and have your friend look for, too!) is your reaction immediately after the "click" of a dummy round. Did your gun's muzzle move? If it did, that means you need work on that flinch because it's preventing you from being accurate! The way to work on it is by doing everything mentioned above, time and time again. Once you learn that the "boom" isn't gonna hurt you in the least, you'll notice that your muzzle stays exactly still when one of those snap-caps sneaks in there!
The reason I say that a .45ACP isn't the ideal 1st handgun for someone is because it does have a big "boom" compared to a little .22lr handgun and that boom and recoil can be intimidating and lead to a flinch. Once the little, quieter guns are mastered, then folks are generally ready to take what they've learned and apply to it to a larger caliber. This doesn't mean that you can't learn to shoot correctly on your XD45, but it'll take practice. It is all mental, really.
