It doesn't take much for us to start rolling downhill mentally.

We need just a few things to not go right, consecutively, for us to start questioning every decision we’ve ever made, our entire existence on earth. Before you know it, we are wondering why we even play this game / started this business / got into this relationship. Perhaps it’s not meant to be? Maybe it’s not worth it?

STOP IT.

100 Mental Game Best Practices: Dominate The Most Important Game There Is
Night follows day, just as Fall and Winter follow summer and rainy days follow sunny days. No one told you it was always gonna be smooth & easy. And you already know this, when you detach from the emotion of the moment and think about it.

Emotions are great at recruiting -- entertain one of them, and it recruits reasons and examples from your life to justify the feeling and soon you’re drowning in it. When the feelings are helping you live productively they're great. The other kind? Not so much.  

You're responsible for checking your emotions when they're not helping you, and putting them back in line. Stay in charge of your mind; when you don't, it could start going backwards.