HOW DO YOU STAY MOTIVATED?

Just returned to MFP, after being gone for 2 months and gaining back 13 pounds of the 36 I lost :(. When I first starting my weight loss journey in July 2012, I was very motivated and determined but like always, I start a project and somewhere along the way, whether is sickness or stress related, I just throw in the towel. I am trying to get the momentum back but my negative thoughts keep telling me "how much I gained backed and now I have to lose the same weight again." :(

What keeps you motivated not to give up?