Tis better to have tried and failed...
CNN published a great commentary by Donna Brazile in which she writes about how to cope with the failure of a losing campaign. Several of her pieces of advice are applicable to the end of a creative project. For instance:
DB: "Acknowledge your success. Think about the nonstop pace that you thrived in but would crush less hardy individuals. You lived for and met multiple deadlines. "
Writers often underestimate how difficult their work is, and underplay their achievement.
DB: "You essentially lived with the people you work with and, God bless you, you didn't kill them, though you probably picked up a few bad habits and gained more than a few unwanted pounds."
She's referring to colleagues here, but replace "the people you work with" with "the characters you wrote about" and it's still true.
DB: "Now you're sitting at that desk and trying to figure out what to do with everything you've accumulated throughout the quest to reach the city hall, the statehouse, Capitol Hill or the White House."
After a writing project is finished, the accumulation of research, character studies, quotes, settings, etc. hang around like people who won't quit the party; the constructed imaginary world you invited into your life and your mind and your heart in the quest to reach publication.
She then goes on to describe the heartbreak of a failed campaign, and it's as if she's describing how writers feel when they get rejection after rejection for their work. The exhaustion, the emptiness, the despair, the "what was all that hard work for?" feeling.
Then she gets to the good stuff. She reminds us to grieve, to mourn, to treat ourselves well, to reconnect with family and friends- that our efforts were not in vain, that the world needs to hear our words. And most of all, she reminds us that the next project awaits.