Do you worry that everything will just fall apart?

My life is actually going pretty well, in general, but I worry that I will fail a class, get fired, get into a car accident, etc.
I almost constantly am concerned about these things (usually obviously irrationally). Currently, I am waiting for a paper to be graded for a class. I am afraid that the fact that it hasn't been done yet is a bad sign (I think grades were due by noon today, and it is now almost 7 p.m., but my instructor graded one thing this afternoon). What concerns me is that I may have not cited something correctly, and I could get in trouble for plagiarism, which I wouldn't do intentionally, but I don't feel too confident in my ability to cite things right. I should have had someone proof read my paper. Until it is graded, I will probably be freaked out about this.

Hello,

You know, many bad things could happen in this life, natural disasters, car accidents, you being mugged...etc

Your apprehensions won't prevent them from happening. The only thing you can do is to lower the odds of these events occurring.

For example : The harder you study, the lower the odds of you failing to your class will be.
You can reduce the chances of having a car accident, by not drinking / being busy with something while driving, paying attention to the road.

My point is, only your actions will have a real impact on the % of occurrence of something , your misgivings won't.
In your case : you cannot do anything about it, what is done is done, stress will only ruin your health, nothing else.