I was about to say this. I can relate hundred percent to the modus operandi, and to be honest I just accepted that I tend to loose interest and that's fine.
Moreover, I came to analyse that what was really interesting to me was this initial dreaming and planning and obsessing phase rather than the long execution.
I tend to discover and learn what I was seeking in the early phase. After that the need is less pressing.
Accepting this I made my profession of being an architect, planner, discoverer, and not a regular maintainer of things.
Note that I also revisit often old stuff and the spark might reignite. I have completed projects in the span of years picking up after a long time.