I know exactly how you feel. I lost lots of weight a number of times starting when I was in my young 20's. I lost over 100 lbs, and I was feeling fantastic. I started going out and partying, and then bad habits took over. I gained everything back and more. Now I am doing this again, but I know I have to do this before my health bites me in the butt. I don't know why we fall back into these old patterns. Sometimes being thin and healthy is a lot more work than being fat, and I think I sabotage myself sometimes. I don't know why, but I am seeking counselling on this issue.
Anyways, good luck to your journey. Don't think of it as a journey for weight loss, more of a journey of getting your life and body back.