There are four hard things in life. If you can do these four things, the rest comes easy:
My brothers and sisters, whenever you face trials of any kind, consider it nothing but joy, because you know that the testing of your faith produces endurance; and let endurance have its full effect, so that you may be mature and complete, lacking in nothing. (James 1:2-4, NRSV)
The poison from which the weaker nature perishes strengthens the strong man–and he does not call it poison (Friedrich Nietzsche, The Gay Science)
Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. (Antoine de Saint-Exupery)
Note: the attributions on these are only a best guess