When I think about it, there is something quite unique about finishing a novel or even a large project. It is for me a feeling of irritability, satisfaction, and relief, almost but not quite an emotional release. The sudden loss of stress work-related always makes me feel just a little bit "grateful" for surviving. Sentiment aide, I do think it's important to take a step back from whatever you just finished to breath and appreciate the moment--your book or project is at long last...done.
it's been said that the first novel is a harsh teacher, your second manuscript is not usually as hard as the first one was. Same thing is true for big projects, especially when you've never done it before. It's one thing to write all the individual pieces of something that gets put together as one big "thing," it's a whole different matter to plot and scheme what all those pieces should be, before you start. When you read this, I'll be working on the final phases of a very larger (i.e., massive) project that took 4 years to start and finish. I recognize the need to calm down and decompress when it's done.
Experience has taught me that some down-time between writing projects is always quite necessary. Most people I have known need as much as 6 months to just "live" after they wrote their first novel. Me? I don't seem to be capable of sitting still for more than 2 weeks. I've been known to unplug the phone and sleep too much, then go back to work after a long scary junk food binge that has been known to last 3 days. Bottom line is this, accept your victory for what it is and then slow down so that your brain can catch up. that much gives your subconscious mind a chance to think ahead. There have been moments when I just can't help myself, I jump off in to the next thing sooner than I should--and--I pay for it later, when I'm exhausted.