I've found that this sometimes happens because I have serious second thoughts about some aspect of the project (maybe I don't know how it could ever make money,

I've found that this sometimes happens because I have serious second thoughts about some aspect of the project (maybe I don't know how it could ever make money, or it seems like it would be difficult to acquire users). The most effective thing seems to be getting someone else involved. That way, you are committing not to yourself, but to another person as well. If that person is a coder, then you don't have to do all the programming work yourself. If not, maybe they can help you to get around some of the shortcomings in your own skillset. Some people just don't work well alone (I'm one of them). It's not that I can't do all of the work myself (I can), but just that I don't enjoy it. I've learned to accept that, and then to get around it by bringing others into my projects at an early phase.