Why am I always nervous, anxious, and distracted?

It doesn't matter what I do. I worry that something bad will come out of every little thing. I worry that I'm messing up my life by missing opportunities or by not meeting the level my peers are on (work, education, etc). I can't focus while studying or driving because I can't stop thinking about the worst outcome of such things.

It's gotten to the point where I don't want to do anything but stay home. I don't, and I make myself maintain my classes, but my heart races when I have to get in a car.

On some days, it even prevents me from getting to sleep because I keep thinking about my 'failures.'

I know everyone gets stressed, but it's constant. No matter what I do to eliminate the causes of these fears, they still remain.