Apart from aesthetic preferences, replacing "loose weight" with "continually increase my fitness" (which is not something you'll ever be done with) is a far more constructive goal (and it will eliminate the diet-mindset-rollercoaster).

I suspect that lack of will power often means impatience. And impatience comes from unrealistic expectations regarding what you "should" be able to do in time n.

Daily moderate (will-power-) exercise will give positive results on a whole range of parameters, but it might take a year before the effects (including the habit of exercising) really show.

Off course you could be an exception.