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.
The tip is implicit.
"Fantasy abandoned by reason produces impossible monsters: united with her, she is the mother of the arts and the origin of their marvels."
-https://en.wikipedia.org/wiki/The_Sleep_of_Reason_Produces_Monsters
That one makes the same or similar claim, but explicitly. Do you get it now?
Another month, another rationality quotes thread. The rules are: