For me, a story is done when one of two conditions is met.
1) The story is as good as I can possibly make it at this point in time.
2) The thought of working on it any further makes me want to cry.
For 1, "at this point in time" is important. I know that a few months later I will find a sentence I wish I'd realized needed to be fixed. But I will go over the manuscript obsessively as many times as possible and fix everything that seems to need fixing before I send it out. This is a lot of work. But it's worth it.
In my experience, every time I've said, "Well, this isn't really a big deal, no one will notice it," every single reader has homed right in on it.
If you find yourself endlessly fixing something and never moving on to another project, stick a fork in it. Either put it away and come back to it a few months down the line (if you really feel it has problems) or send the puppy out (if on every pass you're removing commas and then putting them back next time, or something similar). Move on to the next thing.
I don't know if your cycle of fixing things forever is due to not wanting to move on, or due to problems that you're seeing but don't quite know how to fix yet. Or because you're a naturally slow reviser--there's nothing wrong with that. Either way, putting something away for a few months and working on something else rarely goes amiss. Distance lends perspective and all that.
2 is pretty much self explanatory. There are a few stories of mine that I know have problems but I've sent them out anyway, because I knew that never in my life would I be willing to do any further revision on them. One of them sold, actually, and another one is out doing the rounds. You'll know when this happens, and when it does, trunk the thing or send it out, and forget it.