Theres quitting for the right reasons and quitting for the wrong reasons. If you quit your project make sure you do it for the right reasons, otherwise you'll regret it later when your perspective is brighter. This may be a situation where the business idea is a lost cause and its time to quit (the smart thing to do), or this might be a situation where if you pivoted and tried something smarter, you could have a breakthrough. (Notice I didn't say work harder - thats a fools game).