Fatigue makes cowards of us all. When you’re tired, you feel beat down and vulnerable. You’re uncomfortable and want to quit. Don’t.
What you need to do is practice being uncomfortable. Push through for just a little longer, and when you think you can’t go any further, keep holding on. Practice doing this with small things first (like running just 30 seconds longer, or holding out another 10 minutes before you go back for more food). That way, if you do end up failing, you can recover from it.
In every failure, there is something to be gained. You learn about the situation and you learn about yourself. Now apply what you’ve learned to improve on your efforts for the next time you encounter a similar situation.