Oh, this is an easy one to describe. Nothing that I have ever worked on, as far as I'm aware, has ever felt truly and entirely "done" done done ya know? I don't think "done" exists, so if that's what you were looking for in life, I'd forget about it.
What I have had many times is "done enough". Usually I set a mental target like, "well, this song is going to have x, y and z. So as soon as those are in there and as soon as the mix/master allows for me to hear those parts distinctly..... I'm done enough."
What "done enough" means to you will probably differ, but, generally I would suggest setting a specific and achievable goal for your art and then just f*ckin' kickin' it out of the nest when you get there. It's never, ever ever ever ever going to be "done" anyway1. So hit your goals and hit send and go to the next project ;)
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There's a whole 'nuther discussion here about the fact that 90% of what you make as a musician is going to suck, like, hard. But you just need to kick it out anyway. I may delve into that at a later date but, tl;dr, it really doesn't matter how good or bad it is, just let it go.
