How can I improve my MW2 game?

So, I have been sucking pretty bad on MW2 (360 btw), and I struggle to do better, I did go 17-0 once on skid row (full stealth, suppressed Scar, just a random class I made) but since then I haven't even came close even with the same class. Right now I use the ACR (either RDS or Holo) with Scav, SP, and Commando, but honestly Im dying too much to really use Scav. I was wondering if anyone can give me some pointers with my class? Im thinking it needs to be revamped. I tend to play cautious usually moving while sighted, sometimes I sprint but they always beat me to the punch when I try to sight. I like commando for the whole "no damage after a fall" but as my roommate pointed out, I tend to shoot rather than knife at that range. The other thing is I'm not all that confident in headshots so I tend to aim for center mass (usually hit them in the upper torso, sometimes getting a headshot). Should I change my gun, or my perks? Perhaps both? I just feel like this class doesn't suit me but I have no idea where to start (and just so ya know, I have a nasty habit of over-thinking things).