You've most likely failed on every diet because, not necessarily that you weren't motivated to lose the weight, but because you haven't been committed to it.
It really is about choice. You don't have to be fat if you don't want to be. You DO have the ability to lose each and every excess pound on you. You've got the power, you hold the key. You've got to DECIDE *to do this* once and for all, no matter what and permanently.
You've got to stop dieting and come to the realization and accept the fact that you can't go off and on diets, but that you must change the way you eat - forever.
For me, that couldn't happen until my desire to be thin outweighed, overtook and overpowered the desires for the high calorie/high quantity food. It was time. A change had to occur. The food TASTED good, but left me miserable. And that was no longer acceptable to me. I stopped settling for the short lived pleasure that the food gave me and instead opted for the all day high/good feelings that I get from being slim, trim, healthy and active.
DECIDE to do this, commit to do this, than map out a plan, a mode of attack - and do it. And remember it CAN be done - if you decide to, if you choose to.
I do hope that you choose to, because it turns our adhering to a healthy lifestyle is not the burden I always feared it would be, but it is a joy and a blessing.