Johnny,
The reason why your shots are missing the target is because at some point in during the firing sequence, muscles in your hand are contracting and pulling the muzzle off target. When you fire a shot, you want both your hands to be completely relaxed. The only muscle that should be contracting is your trigger finger.
Instead, one of the following is probably happening:
You're gripping too tightly with your right or left hand
You're flexing your fingers
You're jerking the trigger
You're pulling the trigger back at an angle
You're compensating for recoil
You're anticipating the trigger break
Or it could be a combination of the above. It would be easier to identify if you could tell us where the shots are going.
The reason why you shoot well with the Mk III and so poorly with the 9mm is the difference in recoil. You're anticipating the recoil, and it's making you flinch, jerk, buck or somehow break the rhythm of your firing sequence. It happens to everyone when they first learn to shoot. Here's what you need to do:
First, learn to let the gun catch you by surprise. I know this sounds silly, but the key to a good trigger squeeze is that it should surprise you when the trigger breaks. When squeezing the trigger, many new shooters will decide that they want to shoot NOW, and depress the trigger in one swift moment. That's wrong. What you want to do is SQUEEZE the trigger straight back into you, applying pressure incrementally until <CLICK> the trigger breaks and the gun goes bang.
The second thing you need to do is learn to completely relax your hand when you hold the gun. Try an exercise - touch the top of your thumb nail with the pad of your index finger (same hand). Press down on your thumb nail. Notice how your middle and index fingers also move? When you squeeze the trigger, those fingers are squeezing the grip, which is throwing off your aim. Be mindful of this, and try to keep those fingers as relaxed as possible (it takes practice to break that reflex). You really only need to hold the gun firmly enough to keep it from falling out of your hands; any more pressure than that is unnecessary and will affect your aim.
The best thing you can do for yourself is take Len's advice and go meet up with Jim Conway. I'd shoot him a PM first to let him know you'll be there. Jim's tutelage is a HUGE perk of having a WSC membership.
Second, dry fire, dry fire, dry fire. You should be dry firing 10 times at home for every shot you take at the range. When you dry fire, keep your eyes focused on the front sight. It should not move when the trigger breaks. When you're at the range, dry fire before each magazine, and don't load a magazine until you can dry fire without moving your front sight.
Third, run the ball and dummy drill. Buy some dummy rounds (I like ST Action Pro) and mix them in with your range ammo. Have a buddy load your magazines, or load them without looking so you don't know where the dummy rounds are. Shoot as you normally would (you should be focusing on the front sight normally) and be mindful of what happens when you hit a dummy round. The front sight shouldn't move. If it does, go back to dry firing until it doesn't move. Rinse and repeat.
And keep shooting your Mk III - that's great (cheap) training.
Report back here and let us know of your progress.