How to develop determination and grit?

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I'm a very lucky man, in that I was raised in a (very) upper middle class, conservative town. I went to a very well respected public school, but blew a lot my chances by putting forth zero effort into my classes and extracurriculars.

I've neglected my body for oh I don't know, all my life. I'm twenty now and still look like a skinnyfat highschool gamer.

I thought joining the military would instill self determination in me but it's the same story difference setting.

How do I get fit, stop smoking, study, develop meaningful hobbies? And I don't mean the mechanics behind those things, I know the science behind weight loss and muscle gain, or how nicotine withdrawal affects you and the ways you can deal with it. I mean how do I get myself into a mindset to actually perturb meaningful change in my life?