I don't have a magic answer, but I can definitely relate, as I struggle with this myself. You're not alone.
What would motivate you? That might help make it "worth it" for you to choose what you know to be good for you versus the things you know aren't.
-Giving yourself a small reward after you walk and drink 8 glasses of water for a certain number of days?
-Putting up motivational quotes from SparkPeople like these: http://www.sparkpeople.com/resource/quotes
-Finding someone nearby or on SparkPeople who is willing to say "OK, if you do it, I'll do it, too." Knowing that someone else is counting on you might be motivational.
These are just a few random thoughts. Take care and let us know how it goes.