What is it like to finally be a sucessful entreprenuer knowing that you did all you could?

I would expect this emotion to be different for every entrepreneur. In many ways, it depends on why you're pursuing entrepreneurship. Is it for the challenge? For the financial rewards? For the sense of fixing something that needed fixing? The feeling of accomplishment is no-doubt very rewarding, but why it is rewarding will be slightly different for each entrepreneur.

I'd imagine this would be a feeling best done through experience, and can't easily be described in writing!

I think what it would feel like being an unsuccessful entrepreneur, knowing that you did all you could, would be a fascinating question because it would involve second guessing and soul searching as to what you could have improved upon...