If you focus on self-criticism until you do something about what you don't like, you'll drive yourself forward.
If you don't dare to act even though you are afraid, call yourself a bloody coward. Prove yourself wrong.
If you are too lazy to do something, despise yourself until you can't bear it anymore. Prove yourself wrong.
Then it's a matter of stamina, as well. Psychological stamina.
You know how you do it when you run until you hit the steady state?
Until then, it's bloody hard, but when you reach that mile-eating pace, you can go all the way.
Sometimes you can even go beyond what you needed to do and run another few miles.
Prove you can be more than you are now by being something more.