You are definitely not alone. I have always been big and to me and sticking to my exercise and "lifestyle change" (I hate the word diet. It freaks me out and I tend to binge. lol) is always hard because somehow or another, I lose the motivation I started with.
So this time I've set small goals for myself, not letting the big picture overwhelm me, and I am trying to take things one step at a time. A journal does really help me and I record everything I eat, drink and what exercise I've done for the day. So when you have a "bad day", you can look back and see all the accomplishments you've made.
I also changed the wallpaper on my computer to something inspiring and also on my desktop, that I see everyday, is reasons why I want to lose weight.
For me, it takes lots of little things to keep myself motivated towards the big picture.
I wish you luck!