No one knows what your art is "supposed to look like" so when you give it 90% or 60% as you're saying--it's still 100% because that's what you did. That's what exists. There's nothing to compare it to.
As for regret, only if it didn't reach my intended goal for the project. So if I didn't fit the client's needs, or if I spent waaaay too much time on a project that was faulted on a foundational level and would never pan out.
So going along with that, I think if you're focused on "I must make every project perfect," then you'll lose sight of the forest for the trees. You'll never be able to finish anything--especially if the foundation is flawed. Being able to step back and say "we are done here." by ending a project, or starting over a project so you fix those flaws, or accept "this is about as good as it will be" or to say "this fits the bill" is a skill you have to develop as an artist. It takes time to do that.
I had a teacher who would say that being an artist is as much about destruction as it is about creation--you have to be fully willing to destroy every part of your piece if you want to make it better, and that includes ideas that never happened, details that didn't get drawn, and parts of the piece you just don't have the time for. When you edit out the extraneous, it makes the core piece better, most of the time.
And as for being precious with our art--I think there's also this sense that the art reflects who we are individually--which it does not. Some people get attached to their art and their characters on a deep emotional level, and that can make some really passionate art, but it can also make a lot of unfinished pieces because it must be perfect. But...the art is not a person. It's not real, it's not you, it's just a piece and you'll make many like it. It takes some humility to show a piece that is...unfinished in your mind, but finished in the mind of anyone else looking at it. Because...especially if that's where you store your value, then it can feel like showing your flaws as a human (which again, it is not)