Your story, down to the timeframe mirrors mine. I created a Pingdom competitor (https://www.pingbrigade.com) that was cheaper to operate and cheaper for the users, while being easier to use and supported IPv6. After I launched it I got several hundred sign ups over the two years, but nobody seemed interested in the paid accounts (they let you monitor more than one thing). I did not realize just how attached I would get to this project and how the fact that it never became profitable would weigh on me. Last month I decided to pull the plug on it and sent out an announcement to the users. I got a few responses of "Noooooo!!!! Why???!!" I know for a fact that business development and marketing were not there and that is why it never took off, but it was my flagship project and it is hard to let it go.
Good luck to you on your next project. Hope it does better.