Not really to do with spearfishing as pushing past your limits is incredibly dangerous. But use it in your dry training and it may help.
My brother @Deon_Nivvy sent me this (avid cyclist and did ride elite for a time), he is thinking about getting it tattooed on his back.
Your biggest challenge isn’t somebody else. It’s the ache in your lungs, the burning in your legs and the voice inside you that yells “I Can’t!”, But you do not listen. You just push harder , then you hear that voice fade away and start to whisper , ” I CAN!”. That’s the moment you discover that the person you though you were is no match for the one you really are!