I'm in a similar situation in that after going it alone for a while I'm now looking for cofounders.

Since I haven't succeeded (yet), I don't have a lot of good advice, save this: it's okay to fail, just don't fail the same way twice. From what I've been reading, most startups simply crawl away and die anyway. So the only risk you're looking at is 1) your startup is successful, and 2) the cofounder doesn't work out. Seems like that is a bit of a long-shot.

I'm not saying the selection doesn't matter -- I think it's the critical factor. What I am saying is that you should learn what makes a good cofounder for you, and that's something you might have to iterate on a bit. Don't sit in indecision wondering what the perfect choice is.

That's my plan, for what it's worth.