By ruthlessness I do not mean an attitude of cruelty or hostility to the world or toward other people. You need only be ruthless toward your typical excuses, or the typical non-productive habits that you let yourself fall into. Be unsparing with your self-justifications. Call bullshit on yourself.
*I can't write today because I'm tired / sick / burnt out.
*because it's a teaching day.
*because I got so much done yesterday.
*because I'm not inspired.
*because I have to save up energy for tomorrow.
*because it's not close enough to the deadline for me to feel any sense of urgency.
You see, I already know all your excuses. My Stupid Tricks have a lethal effect on them. You only have your had habits to lose so what are you afraid of?