To cope with doubts and fear I ended up finding a group of entrepreneurs who held a regular meeting. At each meeting we went around the room and talked rather openly about the stuff that was on our mind, good and bad. It was very helpful to me. If you can't find such a group, try to find others like you at networking events and go to lunch to talk things out. I once met a dude who was also a solo founder, in a vertical niche like me, used the same technologies, and even hosted at the same provider! We had some good conversations.

The "everything to do" is a blessing and a curse. Blessing: you're never in a meeting. Curse: everything else. I think the key thing here, if you're going to do everything yourself, is to make a product that isn't too complicated. If your product requires a tremendous amount of engineering work you should get help or you'll never get around to marketing. I speak from experience, sadly.

Wanting to give up: there are two things that get me through the days of poor morale. 1) knowing that if I give up I will have to get a job working for somebody, and all this glorious freedom of being my own boss will go in the toilet. 2) The fact that I am working on something with a workable business model gives me the confidence that it's just a matter of time. My product helps solve a problem, and people who have that problem are usually prepared to pay money for a solution. I think morale would be much harder to maintain if I were trying to build the next consumer-social-sharing app or whatever where the rewards may or may not come, even if I work well.