Oh, it's very possible that it could be confirmation bias, and you make a very good point, Venus. There are plenty of people who are not bipolar that start and never finish projects; my mother is one of them. I remember as a child that she was always working on a romance novel she had on the back burner all the time and over the years she gradually lost interest in it.

On the other hand- nothing kills motivation like feeling tired and listless all the time, or just taking on more than you can reasonably handle. The habit of starting half a dozen things and then losing interest in them tends to fluctuate with my mood cycles, especially noticeable since I haven't been back on medication very long. If anything, after having started a mood stabilizer I have more days where I have the energy to do what needs doing (though sadly that is not the case right now, anxiety is pretty bad currently) so I am actually more productive overall, even with constant low-level depression.

It may not be a cause per se, but I do think it factors into things greatly for me, if that makes sense.