When I was ~14ish I came up with the proto-version of what is now the philosophy by which I live. When I was in college, I discovered that it's almost exactly the philosophy that Alcoholics Anonymous espouses. (which made me wonder how many people I talked to about my philosophy of living thought I was a recovering alcoholic)
It basically boils down to this:
Stuff falls into 1 of 3 categories:
1) Stuff I can't do anything about.
2) Stuff I can do something about.
3) Stuff I can do something about, but don't want to.
If something falls into category 1, then I don't worry about it, because I can't do anything about it.
If something falls into category 2, then I don't worry about it, because I'm doing something about it.
If something falls into category 3, then I don't worry about it, because I don't want it to change.
We took a wide variety of 'stress' tests in my psychology classes, and the professor was always amazed at how low I scored (I never got up into the 'minimal stress' category.
The downside (which took me several years to really realize) is that if I'm not sure how to go about changing something I don't like, then I tend to just accept it and stop looking for a solution. Fortunately, that's where my wife comes in.