How can I stay grounded/rational while working at a startup?

I never thought I'd find myself dealing with startup culture (even after reading HN for so long), but I recently joined a startup at my college (CS senior here). I'm really glad to be working with everyone on the team, as I feel like my college career was seriously lacking this kind of experience. We've gauged some interest from local companies about our product as well as a considerable number of other students. Everything seems to be going well so far, but I guess I can't shake this nagging thought of "people don't know what they want", and that nobody will actually use it despite initial research indicating so. So I guess my question is, how can I continue to look at the product rationally instead of having a warped view of somebody who's put a considerable amount of time stressing out over every detail?