[rhubarbquid]

You've probably built up a decent skillset that would be valuable to a growing startup, if you're interested in working for someone else for a change. Don't be discouraged that you're not an "expert" in any one field... being able to wear a lot of hats is a good thing for startups and small companies. If, for example, you joined a company as an engineer, your experience in other areas like product design will help you work with and understand the product designers there.

Joining a larger company than the one's you've founded might help round out your skills, too.

Most tech startups fail... yours failing doesn't make you a failure. Your other option is to take what you've learned along the way and try again. I've you've previously founded solo, it may be worth thinking about trying again with a partner or two, if there are some candidates to do that with that you've met along the way. They can help with skills and experience you don't have, and can help keep you motivated when you're feeling down.