I need help with mw2 ?! ?

Lately on MW2 I have been doing **** !! Absolutely ****!!! I use a UMP45 with silencer and M1014 and Perks/ Marathon pro, Lightweight pro, Nina pro, martydon, and Semtex and stun and it worked without fail and I would play the best I have ever played but lately I have been doing **** even with the gun I am level 66 prestige 6 and I am just doing terrible and everyone lately seems to shot gun nube tube and camp which pisses me off I know I have a shot gun but I'm going to change it to a pp2000 any suggestions on how I can become better ?!!