My thoughts from experiences over the past 18 months.
Starting something is the hardest step. There's never a right time. Or enough time. Just do it. Inaction is always more costly than being wrong.
Never be afraid to stop something that isn't working. It's never too late to take time out to do something properly. Particularly if you're on a deathmarch.
Reboot. Try again. Change it. Look at if from a different angle. There are never complete failures. See the good that was there. Make something new.