Most importantly, don't despair. Things may not turn out exactly the way you want them to, but they will turn out ok. You live in a great time and have phenomenal skills and experiences. You'll do ok. Never lose sight of that.
It's great that you asked the question. When you're overtaken by waves of emotions, it helps to get an impartial perspective. Here is what I think about your situation. In the movie Lincoln, Daniel Day-Lewis delivers this line:
A compass, I learnt when I was surveying, it'll point you to
True North from where you're standing, but it's got no advice
about the swamps and deserts and chasms that you'll encounter
along the way. If in pursuit of your destination, you plunge
ahead, heedless of obstacles, and achieve nothing more than to
sink in a swamp... What's the use of knowing True North?
You just screwed up with your tactics and fell into a swamp. That's ok -- just make sure to learn your lesson. Deals fall through. No deal is ever done until the paperwork is signed and money is in the bank. When it comes to deals, never put your eggs in one basket. I'm sure you already got it, so there is no use brooding over it. It's time to move on.
The biggest question you need to answer right now is if you still believe your startup is the True North. Is it the best possible thing you could be working on? From your question it sounds like you still believe that. If you do, good. Then it's time to fight like hell. If you don't, there is no harm done. If it's not the True North, what does it matter if you fell in a swamp? You can pull the plug and start again, with the new, truer direction, now based on everything you've learned.
If you still believe in your company, then fight! I don't know the details of your particular situation (valuation, metrics, team size, runway, amount of money you need to get to the next milestone, etc.) so I can't give tactical advice on how to get out of your particular swamp (or even if it's possible). Pick five advisors you really trust. Poll them on what they think you can do to get out of your situation and move on to building your company. Then decide on the best strategy and fight like hell to make it happen.
Feel free to contact me privately if you need to (email in profile), and good luck! Don't despair. Take a day off and do what you have to do to forget about your company for that day. Then pick yourself up and get into work. It's the first day of your company's new life. Where do you want to take it?