There's a concept in BJJ (and I'm sure it's in a lot of physical, competitive sports) where the state of being "tired", is the place where your real training begins.
I heard this today "how you fight when you're tired, is how you will fight on the day" from a Kickboxing video.
And in my experience, this is true. There's a saying I've known my whole martial career (well, one of many sayings), that "Cardio makes cowards of us all" because your will to essentially "play fight" goes out the window when you get tired enough.
You need something more.
More than the game you're playing in a sport. You need to fight something, an adversary to defeat, a challenge that feels like life or death, something that won't let you off the hook. That won't allow you to accept defeat.
And like all things, especially the ones worth pursuing, there is an associated pain.
Today's pain is "I'm tired", and yet there are things to be done.
Do I find something so extreme that needs to be defeated that I can accept this pain of "tired"?
No. I just accept that things need to be done, and I will do them.