I also shot low and to the left this problem arised when I went from a 45 reuger to a 9mm HK usp last time I was at the range I took out a glock 17 and glock 19 ( I just had to see what all the hoopla was about)and i had solved the shooting low problem but I still shoot to the left but it is improving.
What did I do to improve it. I played video game shooters. The gun has no kick bak so it is dry fireing and what I noticed was that when I pulled the trigger the gun would move to the side. Problem was I was squeezeing with my entire hand and not just pulling the trigger. Also I notted that I wasn't pulling the trigger straight back. Now I shoot on the same horizon that I fire on and I think my stance has something to do with the reason why am shooting to the left as three out of five group at what I aim at but usually one to two rounds will veer to the left but not low to the left