You will feel an immense sense of freedom and lightness.
It's pretty obvious why: a weight is about to be lifted from your shoulder. Where your path was a narrow funnel leading to one specific destination, you will now have the infinity of all possible and reachable paths ahead of you. You couldn't do anything but worry about the startup - now you can do anything you want.
The end of a start feels very dramatic before it happens, but once it's done, you will feel a whole lot better. Trust me.
A weight has lifted, and I'm slowly regaining energy - almost feel like I might one day have enough again to start on the journey of another startup.
Sometimes it takes experiencing a setback before you learn this. The confidence you gain from the experience can sometimes be worth more than the gain from everything going as planned.
I'm thinking that's going to vary from person to person. There are a lot of factors involved. Age being an obvious one. For us older folks, who may have an "I only have so many 'at bats' left" mindset, the end of a startup could be seen as the end of, well, pretty much everything.
I know that I, personally, am feeling a bit of a "do or die" thing with my startup. If the day comes that I have to acknowledge that I've failed with this, I'm thinking I am headed to the liquor store to spend whatever money is left on booze, and then driving to Vegas to drink myself to death. My biggest hope will be that I find a hooker who looks something like Elisabeth Shue.
After you've left your first startup behind you, take a break.
Mine was only a month, but it was the most productive, thoughtful, and interesting month ever. I came up with new ideas about business, people, philosophy, and the world, and just had some of the best and most clear thinking I had had in many years. I processed my former mistakes and turned them into valuable lessons that I will remember forever. I had tons of new ideas and started writing them down. I went skiing a lot. That month has proved immeasurably valuable.
It's amazing what your brain can do when it has space, and you need to let it have that space, especially after such a long time being focused on one thing.
My explicit recommendation to the OP: leave it all behind for as long as you are able (a week, 2 weeks, a month), travel, and bring a journal.
And remember—you'll have a new perspective on all of this in a year's time. Try to keep that 1 year/5 year/10 year outlook and just take the next step. Don't worry, it's just life.