This is my second time on SW, and I have to admit, the first time around I was a lot like you describe yourself. I'd get to as far as losing 1st, and then stop counting Syns etc. and give up!
This time around I've lost almost 2st, and don't plan on stopping anytime soon! For me, the thing that keeps the momentum going is having little goals along the way. It can be anything. My last goal was to be in the 12's by the time I went to see Coldplay - I managed that and am now on my way to the 11s! My next goal is to have lost 2st by my 21st Birthday (next Sunday) - I've got just half a lb to go. Then after that, my next goal is to lose another stone by the time I go to Marmaris (3rd Sept.)
Having something to focus on each day reminds me why I am doing this, and what I want to acheive. I want to hit those goals asap, so really tend to push myself. I know that without them I wouldn't have gotten this far.