I found one of my favorites on the random quote of the day site.
I try to follow the philosophy, but I don't have much success at it.
Never bear more than one trouble at a time. Some people bear three kinds - all they have had, all they have now, and all they expect to have.
Edward Everett Hale (1822 - 1909)
I frequently (and mentally) kick myself in the backside for mistakes I made years, or even decades ago. Something stupid I did pops into the forefront of my consciousness, and it's like I just did it today.
Dealing with the trouble of now is a bit easier, except that I often anticipate all kinds of trouble that could be just around the corner. Am I just being prepared? Heading off trouble before it occurs? Or am I taking out an advance of angst on trouble that might happen?