I tend to think no one can objectively judge his/her own work. Creators are inextricably attached to what they create. They can compensate somewhat by putting the work away for a while and looking at it again later, trying to put themselves in the place of someone who's never seen the work, but ultimately, the work has to be judged by someone else (agent, editor, etc.).
We all have days we think our work-in-progress is crap. It may be helpful to discard your worst and best opinions of your work and just push on to a draft you think is worth submitting. The point when you can't think of what else to revise may be a good time to submit and let your agent/editor advise you from there.
I've heard many writers say the more they write, the harder it gets. This makes sense for writers trying to create new situations and new characters with each new novel, trying to make it seem as if their series protags are encountering new people and places. Practice only makes perfect for repetitive tasks like trying to write more or less the same book over and over.