Flobble - you've hit a stage in your life, where this time you will suceed. For years I've wanted to and tried to diet, but something clicked earlier this year, and although it's been tough, my motivation has been stronger.
I think you're there. The click. Even if setbacks and slips are put in your way, you are still determined to suceed. Thats why it will happen this time. I also started in morbi-obese, now I'm 21lb of goal, near the middle of the healthy bmi range.
It can be done, I didn't do it all in one go though - couldn't have, those that do are amazing, so you are right not to beat yourself up over what sound like minor slips. I slipped and am still here, and nearly there ..... there's your motivation.