Three ways I know of to motivate yourself are sublimated anger, sublimated sadness, and flowing from faith.
To sublimate anger:
develop the anger, direct it towards a wholesome goal. clean up the mess.
This is foolish in the extreme in my opinion. It leads to ugliness, foolish mistakes where you think you are winning when you are actually losing and vice versa. It also destroys metta and generally makes happiness unatainable. It will motivate you for a while but the price is far too high.
To sublimate sadness, use the sadness to think one of the following into your sadness.
“When will I become a Buddha?” (most difficult, highest motivation)
“When will I become an arahant?” (close enough for me)
“When will I spend the rest of my lives in heaven?” (least motivation I can approve of)
My attempt for Christians (untested possibly dangerous) “How can I attain the heavenly rebirth?”
Finally, there is the motivation that comes flowing out of well tuned faith. The most basic faith is the faith that you can accomplish the present goal. The most useful faith is confirmed faith in the Buddha, Dhamma, and Sangha. see this local article for more on this.