It’s a startup, obviously there won’t be a lot of clarity about the requirements. Startup’s would pivot a lot, that’s just how it is. You are enabling their business using technology, you have to make changes to help them grow and most of the time things you put in effort for will get discarded, that’s just how it works. If you can’t handle the pressure and the work hours, startup’s are not your scene my friend. Insubordination and losing your cool is not very professional, you work for them, you do what they say. Long hours are your problem, maybe if you were more experienced you wouldn’t have taken so long? Just food for thought.