The hardest part

What is THE hardest part of parkour/art du déplacement? Knowing when to stop. Once you learn how to push yourself, it becomes very hard to admit that you must rest _now_, when your friends are still training. I am not injured, but if I continue today- if I continue now- I will break. So… mental strength now, so I can continue (forever. The morning I do not awake, is the day I quit!)