I just came across this classic post on sherdog, posted by HomerPlata in the thread Mental toughness challenged by BJJ?:
"I agree that grappling challenges your mental toughness but, like some others have said, not in terms of resisting locked-in submissions. It's more about whether or not you can drag yourself to class when you feel like the weakest, most talentless turd on the planet. Everyone has those days, and if you can still get to class, listen well, and train hard, then you do have mental toughness."
I can't vouch for being mentally tough, but I certainly have that weak, talentless turd feeling at times... Haha.