I find that if random people on the internet like to use what I'm working on, that's the most motivating thing. Think of it like fishing for startups. If you get a lot of nibbles on a particular idea that you test out, best to keep going if it's working, provided that's it's the best idea you have. If random strangers are happy to get on the phone with you to ask you questions about whether they can use your product because they want to learn more and have a problem, that's a good sign too.