Call of Duty: Modern Warfare 2, why is it so hard?

I bought the game, and even at the lowest difficulty level I'm being blown to pieces. I haven't played these kind of games in 5 years, but I've played games like Quake 3 at the highest difficulty level. Here, however, I don't stand a chance. In the favela mission in Rio, there are simply way too many enemies - I just can't simoly kill tjem and move on. They keep coming and shooting me, up to the point't that I'm frustrated and fed up with the game altogether. Any tips?