Not by our circumstance but by our disposition

From Success and Happiness by Phil Bowermaster. The article actually focuses on the chicken and egg conundrum of happiness and success; which leads to the other? In doing so though, the author quotes the first First Lady, Martha Washington:

I am determined to be cheerful and happy in whatever situation I may find myself. For I have learned that the greater part of our misery or unhappiness is determined not by our circumstance but by our disposition.