I'm always insecure about my work, but there comes a point with each book where I think, "Yes, I like this. It's ready to go out the door." Sometimes it's "YES! I love it!", sometimes it's "Oh hell, this is the best I can do with it, so I'll throw it out there and see what happens."
I've reached that point with 4 books...1 is now contracted, 2 have gotten enthusiastic requests for partials, and the fourth was only recently queried.
When I feel like I'm spinning my tires with something and I can't put my finger on what's wrong, that's when I toss it to a beta reader and work on something else for a while. The usual result is either:
"Hmm, it's okay, but..." (and whatever follows the "but" usually makes the lightbulb come on and I fix it)
or
"I like it. Especially..." (and whatever follows the "especially" is usually something I thought was stupid or didn't make sense. Go figure)
Your mileage may vary. I find that insecurity is par for the course, but that "YES!" moment does come. Sometimes it takes a while...you'll know it when it happens, and it'll be worth it. (For what it's worth, it took over 10 years of learning the craft, 3 full drafts of my trunk novel, and a first draft of a romance novel before I finally had that "YES!" moment...be patient.)