Just wanted to say that I exhibit much the same set of phenomena that you do. I don't ever finish anything I start, I always look for something new, I ride the wave of exhilaration that comes from sinking my teeth into some serious new intellectual problem, coding 18 hour days, and then give up entirely a week later and endlessly refresh HN. I quit jobs and relationships etc. all the time, and then strike up new ones which seem to be destined to last forever.
The difference is that it doesn't bother me so much. I'm quite happy with my life as it is. I think that the need for "production" and "accomplishing goals" etc. is largely a cultural artifact. It's not really important. I think it'd be incredibly boring to be doing the same job for a whole two years.
It stopped bothering me roughly around the time I started exercising (martial arts) and meditating (zazen), so perhaps that has something to do with it. But that's also the time I graduated from school, so maybe it was the newfound freedom that made me happier. Note that I didn't change one bit - I still am a start-only person. It's just that I'm comfortable with it now.
I think you should just enjoy it, if you can.