I used to have this problem. In my case I think it boiled down to the planning fallicy.

That's a really good point. The first project looked to have some cost X and some benefit Y. Once you start it, all the "unknown unknowns" come out of the woodwork, increasing your estimate of the cost. Let's say to 3X.

Meanwhile, shiny new project comes along, and you estimate X as the cost again, with payoff Y. So even if you pay attention to costs, if you fall prey to the planning fallacy, it can look like the new project has a better cost benefit ratio.

Have people noticed a potential link between the planning fallacy and akrasia before? After you fall prey to the planning fallacy again and again, you will associate working on your problems with an increase in your problems. How could you ever be motivated to work in such a world?