INTPs have these constant internal battles. We're good at concentrating and love working in the realm of ideas. However, we're often an impractical lot and procrastination is pretty rampant among the other INTPs I know. The biggest problem, I think, is that we just really suck at perceiving the passage of time, and the daydreaming, writing, or discussion of ideas is often rewarding enough just to stop there.
INTJs are supposed to have many of the same qualities of INTPs but tend to be more sure of themselves, and therefore, more productive (or more capable of delivering 'products' within 'deadlines'). They seem to live in the moment a bit more, and I would venture to guess that they make better entrepreneurs.
As an INTP, I feel I can relate to your situation. I too often wonder if my problem relates to a personality temperament, and truly can be changed. It really bothers me that I have little to show for all the work I do in my head, and this frustration has helped motivate me, but I still don't feel like I've been truly 'unlocked'.
To compound the problem, INTPs make up about 1% to 3% of the population, which means not many people can relate to the INTP mindset and are more likely just to call you a whiner without attempting to appreciate where you are strong, and why that strength makes you weaker in other areas.
Any INTPs out there who feel they've overcome their temperament's negative traits? How did you do it?