Ask HN: How to find a project we enjoy again?

My best friend and I have been working on different projects together for more than 8 years now. Our last project was fun, but we didn’t reach our goal (1 paying customer in 3 months), so we decided to quit it. Now we are both enthusiastic to work on another project, just for the sake of fun, but somehow we have become very critical and don’t like any of our ideas.

What should we do? How can we find a project that we can enjoy again?