That's not even true, though.
It's not true, epistemically, but is true instrumentally.
I explicitly wrote it was an instrumental rationality quote, not epistemically rational because of the kind of literalism you've so gratuitously supplied :)
I don't think it's true, period. It's true that "No one is in control of your happiness but you", but it does not follow that "therefore, you have the power to change anything about yourself or your life that you want to change". And for it to be true instrumentally, it should give us a tip about how to control your happiness.
Another month, another rationality quotes thread. The rules are: