Archimedes is right - don't beat yourself up. But also, remind yourself WHY you want this. Find a way to keep your head in the game.
Here's my little secret. Tumblr. Seriously. I have all Fitness blogs that I follow and before I even get out of bed in the morning I look through them, taking mental nourishment from all the great plans and attitudes. If you can keep your goals in mind, using whatever works for you as a reminder, the motivation is easy!