Imagine your life is all that it could be. That you've accomplished everything you've ever dreamed.
But each of us habitually fail to achieve our true potential. Whether through disappointment or perceived failure, giving up seems to be all too 'the Human condition'.
You should never say, "I can't never" 'cause that's a double negative meaning, "I can ever" and no one will really know if that's what you mean or you're just illiterate.