Sometimes in life the truth hurts. Maybe it's a girl you love who makes it painfully clear she doesn't give a fuck about you but still wants to use you. She wants stuff from you and to 'be your friend'. Maybe it's a drill where you perform like shit. Maybe it's a fight (training or real) where somebody kicks your ass because they have better skills or are more physically capable or better conditioned.
This all sucks. The thing is you can be a wimp who goes home and cries or you can learn from it.