Finishing anything you're working on is a bittersweet experience... on one hand you experience the relief of having it down, excitement of holding it, on paper, in your hand, still warm from the laser printer. But, then, there's the realization that it is, in fact, over, and the self criticism kicks in and the realization that it's much different in execution than you imagined it in conceptualization.
Of course, this is all part of the process... and it doesn't need to be soul crushing, as an experience.
I'm thinking about it because I finished a screenplay that I'd been working on for a while (just last night). I went through the range of emotions. It was... emotional.
I need to put it away for a while because I'm rather angry at it right now for being done. I'm sure I'll get over it, of course... eventually.