I get energized about my side project when I get suggestions, comments, or other feedback from users. I can go months without doing anything and then I'll get one person asking for a feature or change and I'll work for a few hours or days to get it done.
My advice is to make is easy for customers / users to give you feedback. Use a support ticket system and provide email links on your homepage.
Not just bug reports and feature requests. Compliments do a little wonder in fueling motivation, so the more things you give people to comment on, the greater the chances of hearing back will be.
Keep a public work log, publish your progress and encourage feedback. Not just commit history on GitHub, something a bit more humane and substantial. Something to look back and see how much you've accomplished so far.
My advice is to make is easy for customers / users to give you feedback. Use a support ticket system and provide email links on your homepage.