I wouldn't say that I have been developing "work ethic", but I would say I have been actively developing "work capacity" in the last few years. I was diagnosed with a genetic disorder late in life, which explains a lot of my lack of energy, focus or accomplishment over the years. But I have also been on a few lists for parents of gifted kids and this kind of issue comes up fairly often as a topic: That gifted kids are frequently kind of lazy and used to being able to spend 5 minutes on something and get A's. Many of them have a crisis when they hit college and have to really study for the first time in their lives. For one thing, many of them don't know how to really study because they never really had to. For another, it's a huge shock and identity crisis.
I didn't go through that with college. I had good study habits. I did find college was actual work, much more so than high school, and I did have some bumps in the road initially, but nothing like some of the stories I have heard. However, I was quite ill when I was finally diagnosed in my 30's with "atypical cystic fibrosis" and it rapidly became clear to me that I could get healthier but not if I remained married to the man I was with. Like all my doctors, my ex was all too happy to blame it all on my genes and wash his hands of any real responsibility. It was clear to me that he would never get on board with the drastic measures needed for me to get healthier and that if I stayed, I would face the horror of dying a slow, torturous death essentially at the hands of loved ones and doctors who would all think I was insane for blaming them. It was clear to me that leaving was my only hope of getting well and getting well was my only hope of proving that I was right, this could be done. So I got divorced while very ill and started my life over.
I got a Certificate in GIS before divorcing and had a good call back rate on my resume. But I was very ill and doped to the gills, so I didn't interview well. I ultimately ended up with an entry level job unrelated to my training which paid about half of what the GIS jobs I had been applying to paid. I've been stuck in this same job for 4.5 years while getting well. Being so sick has hampered my performance and prevented me from getting promoted. So I have had ample opportunity to get over thinking I'm all Special and can just slap something together at the last minute and it will still be vastly superior to what everyone else is doing, which is my historical experience prior to trying to start my life over while very ill.
FWIW: A couple of very intelligent people at work who went from basically the job I am doing now directly to a supervisory position later got fired. I think a lot of very talented, intelligent people have serious, serious issues with what you are calling "work ethic" (or ethics of any sort). They think the rules don't quite apply to them because, in fact, the rules for someone very talented really are different. I think such people are pretty routinely in danger of getting confused about just which rules they really have to follow. I am aware that I am often able to find a loop-hole or what not -- that doing something well enough or doing things differently in the right way can put you in a different space -- but I am also acutely aware that social and ethical stuff (like respecting other people) still applies, no matter how smart I am.
I would suggest you a) look to your health for a possibly unrecognized, and thus untreated, issue (such as allergies) and b) put yourself in a situation where your natural talents aren't terribly helpful and you just have to do the grunt work, like everyone else. You might try volunteering in some capacity to accomplish this piece of it. I learned a lot of humbleness volunteering to teach math (and tutoring a mentally retarded girl who was in algebra and worked harder for her C's and D's than I ever worked for A's and B's) and doing volunteer work at a homeless shelter. Privilege tends to foster laziness. Getting out of your little insular world (if you have such, I don't know if you do) can be wonderfully growth-producing.
EDIT: As for the health stuff and my focus on the idea of "work capacity", please note that Will Smith has to have a certain amount of physical work capacity to make the statement you quoted. So if you don't have that attitude, it's possible that it's because you just don't have the physical capacity -- i.e. you aren't as healthy/fit as he is. If you want to be like him in terms of attitude, I strongly recommend you work on developing work capacity, as I have done, which is very much rooted in physical ability to do sustained work. Even minor health issues, if not properly attended to, will interfere in this regard.