So much in this business is relative. Whether something is "good" or even "publishable" comes down to people's opinions, in the end. Even after a book is published, it's success is, ultimately, based on a combination of luck, marketing, and (reader's) opinions.
But the first opinion to count in writing is your own.
This is something that has taken me years to figure out, in part because I'm stubborn and in part because I'm dense. Because when I first started writing, I very rarely consulted my own opinion on my work--I very rarely considered my work with a critical eye. I was more concerned about getting the words on paper, and, at least in the long, long ago, the words on the paper were good merely because they were words on the paper.
I had a different definition of "my best work" when I wrote my first novel. It was best merely because I wrote it. I revised it based on grammar only.
I see this all the time in my position as writing and literature teacher. Students write something, and when I ask for a revision, a few sentences are swapped, a few commas deleted. Restructuring an entire essay--which might mean cutting one page and re-writing three--does not even occur to them, even with my big red ink pen slashing away at their work.
To them--and to me, if I'm honest with myself--I don't see how my work can really be better than it is. I think that it's my "best" because it's there.
It's harder to realize that this applies to one's own work, especially when one's own work is a 70k novel that's already been re-written twice. It's something that, for me, takes just time and practice as I train my eye not to think about the mood I was in when I wrote a passage, and to analyze the passage coldly in terms of plot and character. I never thought I was one of those sentimental writers. I always considered myself willing to ruthlessly kill my darlings. But it's more than just cutting a well loved first sentence to re-write chapter one and make it flow better. It's about stepping back and looking at your work not as your work, but as a novel not written by you--a novel that needs to be broken and glued together and polished until it shines.
Segue... My parents found a piano in a barn. It's at least a hundred years old. When I was little, they had the piano's insides worked on and keys replaced, and I learned how to play on that piano. I play better on it than any other piano. All those fancy pianos where you don't have to stand on the forte pedal or where high C doesn't stick--pshaw, those are for city-folk. My ancient instrument trumps theirs any time.
But, if I wasn't so close to that piano, I'd be able to let it go long enough for a real refurbishment. I'd get the wood restained instead of trying to save the finish with wax.
That's what I'm coming to realize with my book. If it wasn't mine, I'd be more willing to take out that chapter where the characters don't do anything. I'd be eager to re-write the jump-the-shark ending that I don't even really like in the first place. I'd be happier with changing that other character's motivations and background to better fit with the story arc.
In the end, I know that I will eventually make those changes. It's one reason why I'm already starting on my third re-write...I have to make those big changes one re-write at a time. And although I don't think I can ever let a furniture restorer touch my piano with sandpaper, I do think I might be able to kill my darlings a little bit more, until it really is my best work.