Dorothy Dix, responsible for the statement, was an American journalist. At least, that was her pseudonym. Her real name was Elizabeth Meriwether Gilmer (1861-1951)
I don't know if I agree with her or not. At the moment, I'm very busy and it's all going well and I feel in control of it, happy and fulfilled.
But sometimes when I'm very busy, too busy, I simply can't cope.
DEFINITELY not happy then.
I don't even know where the dividing line is between very busy and happy and too busy and unhappy.