I'm not sure how productive it is to lump yourself into social categories like this. I tested as an INTP as well, but I would venture to say that I am generally a "finisher" (at least, when it comes to my projects). When I am really drawn into a project, I lose myself: nothing else matters, and I meticulously perfect each and every detail until it is "finished". Then I drop it and move onto something else.

I empathize with the other characteristics you mentioned: "dawdling, distracted, and forgetful of mundane chores, late for obligations, losing homework or library books, and generally disconnected from the external world". But I make a point to finish my projects, or else I feel dissatisfied with myself.

I think part of how I do this is that, although my attention does wander, when I am working on a project it only wanders across little details, rather than large concepts; rather than getting bored with the project itself and moving onto another, I merely get bored of a detail and focus on another (usually completely unrelated, but still within the realms of the vision that is the finished project). After doing this for a while, eventually the entire project is finished, because that is really all it is — a collection of little details. (Of course, this only works if you are perfectionist as I am.)

To be honest, I think that these arbitrary measurements like IQ tests and personality descriptions are irrelevant (and maybe even procrastinations) to solving your real problem. If you want to be able to finish projects, just work on them — you won't know what motivated you in theory until you discover it in practice.