3 weeks level 52? that't not bad, especially if you're playing team deathmatch, i play mainly search, domination, and team deathmatch. Search is easy points, domination is a lot of kills and points, and team deathmatch is just good ol deathmatch. Try to slowly raise your sensitivity, i started on 3, now i am on 8, its frowned upon, but camp until you feel comfortable to go out and look for the action.
As for classes, my favorites are...
UMP45 Silenced
Spas-12
Semtex
Marathon
Cold-Blooded
NINJA (Pro especially)
^This class is for basically running and gunning, but not showing up on the radar. The UMP is the best gun and being able to run infinitely and not be detected is straight cheating, but hey its in the game!
Scar-H Red Dot Sight or Silenced/TAR-21 Silenced
Akimbo G18 or Akimbo Rafficas
C4
Sleight of Hand
Cold Blooded/Stopping Power
Steady Aim
^This is a basic loadout that will work for anyone, both guns don't need stopping power, but if you're still getting murked, stopping power helps, akimbo with steady aim with the G18/Raffica's are an automatic kill within 10 feet, and still reliable mid range.
These are the classes that would definitely help you get better.
So try to get a higher sensitivity, go for lower kill streaks...UAV/PREDATOR/HARRIER...UAV is VERY important, knowing where your enemy is everything
If you're playing on the PS3, add me Bunnies_R_Cute let me know you're from NIKETALK and we'll get some team deathmatch going on