My extra heart is not available when I lose my last heart.
I thought the extra heart when equipped is supposed to be usable when you get down to your last heart, but each time I want to use it today, the button for the extra heart disappears.
You need to use your extra heart prior to actually failing a lesson. It cannot save you from a lesson that you have already lost. Are you trying to use your refill when you have 0 hearts, or when you have 0 hearts and answer another challenge incorrectly?