And that's going to involve the "D" word.
The boring, dictionary.com, definition of self-discipline is pretty self-explanatory: discipline or training of oneself, usually for improvement.
My definition, though, is: the triumph of goals over "What I Feel Like."
Just stop for a minute and let that sink in.
Self-discipline is getting up early in the morning to work out when what you feel like is staying put in the warm, soft, inviting bed. It's biting your tongue when what you feel like is letting loose when your child does something that makes your blood boil. It's calling the insurance company about that charge when what you feel like is not bothering. It's slogging your way through your train wreck rough draft when what you feel like is throwing it out the window and working on the new, shiny, so-much-better idea you had the other day. It's not checking email 15 times a day when you feel like procrastinating. It's turning off the internet for your writing time. It's writing instead of watching TV or whatever other activities you are giving up for your dream.
Self-discipline is giving up what you feel like doing now for a future worthwhile goal.
To quote Gary Ryan Blair: Self-discipline is an act of cultivation. It requires you to connect today's actions to tomorrow's results. There's a season for sowing a season for reaping. Self-discipline helps you know which is which.
It's often hard, but once you discipline yourself, you can do almost anything.
And if you're the affirmation type, here is a blog with a list of self-discipline affirmations.