How do you know you're ready?

So many editors, agents and published authors give the advice of "Don't submit a manuscript until it's ready." Well...how do you ever really know?

I'm in the middle of revising an old manuscript. This MS has been revised before. This is probably the second or third time. This MS has been out on submission twice. The second time was after extensive revision and probably a year and a half's time. Both times I sent it out, I truly thought it was great. I had worked on editing it for months. I'd gotten tons of feedback from writers group members and beta readers. Everyone thought it was great.

But as with anything, the more you do it, the more you learn. So of course I know more now than I did back then. And yeah, this most recent revision has fixed even more problems. Once again, I think it's great. I'm hoping to finish up one last read through today before I send it to beta readers.

But, I've been here before, thinking something is great, but it's really not.