Why do I suck at Modern Warfare 2 so much?

Ok heres the deal i’ve been playing cod for quite a while now, im lvl 26 first prestige, but my kill death ratio is only 0.74, my highest killstreak is 19 and yeah i generally suck at it. So any tips you can give me to improve?

Extra details-
-I mainly use the UMP45, and usually silenced, with sleight of hand pro or scavenger, stopping power, and commando. I only use assault rifles and snipers on a few maps, and for them I use either M4A1, or ACR, and the Intervention for sniper
-I dont camp
-For eqipment usually semtex or throwing knife

Thnks for the help, I still find the game fun without being good, but I get bagged out sometimes for sucking at it (I have no idea why, i mean its just a game), so would appreciate some tips