Why do I suck so much at MW2?

Stay inside buildings when the enemy has aerial killstreaks e.g. Chopper Gunner, don't stay in one spot for very long and learn the spawn points for each map. For your classes, get rid of the silencer on the Tar-21, replace Claymore with one of the grenades (unless Claymores get you a lot of kills) for both classes and use Sleight of Hand Pro, Stopping Pro and Commando Pro (if you think commando is cheap, then keep Steady Aim Pro) as your perks. Hope this helps.