I've read about people who have lost lots of weight with Slimming World, but you can lose weight on any diet, it's just that with slimming clubs, there is a support network there to spur you on.
At the end of the day, weight loss is down to will power and self discipline. If you don't feel it deep within you to make life changes with your eating, you will keep falling off the wagon.
You might find things easier if you don't look on weight loss as dieting. Adopt healthy eatiing habits and start some gentle exercise 20 minutes 4 times per week to start.