Gratitude is part of the puzzle; the other main part is being kind to myself. I have cranky, sad and lethargic moments too. But instead of criticizing myself at those times OR faking gratitude I have experimented with another practice backed by science.
Of course my efforts to please my daughter didn't make her happy. They only made her appreciate me less. While she was mild-natured and agreeable with babysitters and teachers, she became grouchy and oppositional with me. She was only four, but to me it felt as if she was going on fourteen.