It doesn't take a special diet, pills, or even massive willpower. Losing weight and keeping it off takes a commitment to the path and never, ever giving up. You say you have no willpower - but that's a lie.
You're making choices every day, and it is as simple and as hard as making a different choice. A health promoting one.
And then taking the next choice and making that a good one, too.
Weight loss and management isn't perfection, but it is the accumulation of the small decisions we make every day. YOU control what you put in your mouth. YOU are in the driver's seat. Not Slimming World, not motivation, not metabolism or anything else. Pick a plan - preferably a balanced one that suits your tastes and lifestyle. Then commitment with ironclad determination, to do it. Then you do it. And keep doing it. And never, ever give up.
Will you have bad days? Yeah. Will you make poor choices? Sure, none of us is perfect. But the difference between a perpetual failing dieter and a successful maintainer is nothing more magical than sheer stubbornness to see this decision through to the end of life and treat every decision as important. Every day is new, every choice is another chance to work toward health and succeed. Have a bad meal? Good news - the very next one can be excellent! You can start right this moment, without even a solid plan, making health promoting and positive choices. Choose better takeout options, get off the couch for a little workout. Make a list of things you can do to improve your health and self esteem. This is ALL under your control.
Don't give diets and elusive motivation power of your life. It isn't a method or an emotion that makes you succeed, but conscious choices and the overriding attitude with which you approach your life. I'm not always motivated to lose weight - but I am committed to doing this and never giving ups whether I feel like it or not.
Being miserable isn't enough to make that decision. Seeing your health deteriorate and hating your choices isn't enough, either. You must be ready - truly ready - to take responsibility for yourself and choose a differ set of choices each and every day. This is why diet attempts fail - it isn't the willpower of the dieter, or the mechanics of most plans - but that they are being attempted by someone who isn't ready to commit to the lifelong changes that would give them lasting success they desire.
Examine your own heart - if you are truly ready to make a change and lose the weight (and keep it off forever), if you are ready to regain your health - then decide to do it. No backing off and no reneging allowed. All that's left after that is taking each choice captive and consciously moving toward your goal, bit by bit. It is the sum of those small parts that makes your success, not any one make-or-break effort. Keep trying and never quit, you'll surely improve your life and health if you never give up.
And that's my two cents, from someone who wasn't ready to make that commitment for most of my life. But once you're there, all that's left is the journey