I find that to be truly motivated the source must be intrinsic. Extrinsic motivation, the carrot and sticj kind, will only last for so long. Just until the punishment fades from memory, or the reward is already given. But intrinsic motivation is the kind that comes from our passions and loves, and its power is unlimited. It can keep us going through adversity, exhaustion, and even through circumstances that might cause the loss of hope in others.

In other words, find what inspires you, what ignites your passion, what causes you to love deeply. Once you find that you'll have no lack of motivation.