I think to stay motivated it's important to validate your believe that it's worth building. Otherwise you'll find doubt creeping in. If you're making something for other people, then you should show other people what you're making, and get them using it early on. I find there's nothing more motivating than other people being interested in what I'm doing, asking for XYZ features, and telling me what they're trying to do with it.
Also, try to work quickly towards a minimum viable product. It helps to have something to show for your effort as early as possible. Until then, all you have is your imagination, which is much easier to be sceptical of.
1. Base your idea on research. https://stackingthebricks.com has lots on this. But basically if instead of "I HAD AN IDEA" your process is "read read read read read WOAH these people have a PROBLEM", you have evidence that your idea is useful to someone. (My audience for my product is programmers. Research led to blog post, which ended up on front page of HN, created another blog post which again made it to front page, and then I started work on product. Made a big difference to long term motivation.)
2. Have more than one motivation. Sometimes motivations can be contradictory: "learn new technology" is bad motivation to pair with "build a business" because it slows you down with irrelevant details. But "learn how to do content marketing", say, is great paired motivation because it both helps you with feedback loop and means when you're stuck on building you can switch to something else, keep going, and still feel you're getting value out of your work. Longer version, with slightly different focus but same basic idea, here: https://codewithoutrules.com/2017/08/03/stay-focused/