calling it finished

what i think is a very hard task is finding a point in which you are finished with a painting and can hang it on the wall and call it a piece of art.  yesterday i worked on the same painting for an entire day and having worked on it previously.  i want to call it finished but it is still missing something.  it's a little frustrating to have to look at the same work day in and day out.  i've started other paintings and yet i still go back to this one with uncertainty.  i even accidently poked a hole through the canvas out of frustration.  it seems the artist always knows though, when a piece is finished.  i think a lot of work i have hung in galleries is not finished and the only reason i hang them is to have something to hang.  it is very hard to deal with but that is the downfall of showing in a gallery to represent yourself. there will always be deadlines you have to meet.