I truely believe that if I had that answer, the exact, right answer, I would be a millionaire. We have all struggled with this. No matter what our reasons for wanting or needing to lose weight, sometimes they just can't get us to do what we need to. I have lost 55lbs once, hitting Lifetime at WW, 31 lbs once, 20 lbs another and each time I thought that I would never ever go back to the way I was and the next thing I know there is a candy bar in my hands and I'm back where I started. I don't know the answer, but I guess there are things that help to start the process. Having depression doesn't help b/c it zaps motivation. I would start there and try to do everything possible to treat that and at least that may level the field a little. It usually helps me also to take some time and make a list of all of the things I hate and have missed out on being fat and unhealthy all these years. Really feel the burden of it all and remember that although it doesn't seem like it, we alone choose what goes in our mouths. Sometimes I also try to be thankful that I have a problem that I can fix, instead of an ailment that I have no control over, such as cancer or a chronic disease.
Just keep reading and posting, your sure to find some inspiration, empathy, and plenty of folks just like you.