Why am I so bad at MW2?

It is the only game I play, I have probably spent 20+ days playing but I don't know because I got deranked. Yet, I have around 10 kills and 20 deaths in every match I play. I even don't do well using tryhard classes like M16s and TAR-21s. I don't understand, I used to do well and drop chopper gunners all the time, but now I'm so bad I can't even shoot a guy who's not looking at me.