I've felt like this in the past and it's a tragedy of modern times that so many smart, hardworking people feel this way, not just about software but their careers in general.

I agree that having no users has caused some or even most of your anguish and that needs to be fixed but I disagree that joining a large company with customers will necessarily make you that much happier in the long run. In the short-term it may help and perhaps it's something you should try but down the road you'll start to feel like an inconsequential small cog in a machine and possibly end up back at this state of mind again.