There are a total of five different difficulty settings in the new Modern Warfare game, and when you first begin the single player campaign you get to choose one. The five difficulty levels you get to chose from are the following:
Recruit: For players who are new to Call of Duty.
Regular: For players with some FPS experience.
Hardened: A somewhat difficult difficulty that will test your skills.
Veteran: A challenging difficulty that requires a lot of skill.
Realism: The most difficulty difficulty for those who really want a big challenge. There is also a very limited HUD/UI.
So, how do you deal with it if you chose the wrong difficulty after having completed a few missions? It's actually very easy to change, and you can do it at any time. All you need to do to change the difficulty in Call of Duty: Modern Warfare is to open up the main menu, then click on the Options menu. There is a setting for difficulty in this menu, so click on this and choose the new difficulty you want to play on.
And that's it! Enjoy the new difficulty, and feel free to change it again if it still doesn't suit you.