I think you can be a succesful solo founder... but you need some kind of positive support group or network.
Especially if it's a "niche" or lifestyle kind of app (as opposed to the take-over-the-world plans VCs prefer to fund).
By support groups or networks, I'm thinking coworker spaces, accelerators (like YC itself :) ), meetups or other places where you can network, bounce around ideas, get motivated, etc.
And maybe you'll find a cofounder there :) , but I'd go to meetups and events with a very different mindset - just discussing ideas and learning from others and generally motivating yourself :) .
What I've heard is that co-founders help smooth out the ups and downs in motivation and the emotional state that comes with the startup roller coaster.
I never tried quitting my job to be a "solo founder", but I did try several side projects. All of them failed so far:
For my latest "failure" (which might revive :) ), I teamed up with 2 friends which were also working full-time, and we didn't agree on the direction, the tech stack, etc. and we didn't "put in time". I'm especially guilty since I was supposed to be the leader.
I've also failed teaming up with family members (which expected to be the "idea guy" and let me do all the legwork). The ideas were good, but they weren't available and didn't want to commit their time.
And mostly, I've failed on my own, starting up a project, and letting it die by not commiting time to it. Lack of motivation. If I'd put myself out there and validated them, maybe they could have grown. So, I've failed as a solo "founder".
I'm cautiosly optimistic about my current side project, I teamed up with a coworker, we see each other every day so we can prod each other :), we agree on the tech stack (what we do everyday), and have a similar vision. We also have a prospective customer and beta tester :) who was the one that initially expressed the need to be solved.
Besides being a second set of hands to work on the project, I really appreciate his perspective and motivation. That's what I get from a "cofounder".