Wow, you are really spiraling! Wait - take a breath.

I'm just going to make a list of things to consider:

• When you attempt something that is N+1 of your ability, it is going to be challenging. If it was easy, everyone would do it.
  
• No one feels like an expert, EVERYONE feels like a fraud - well into late adulthood. It even has a name: Imposter Syndrome If you see people that appear confident, they are putting on the same front as you are! I'm sure your family and friends think you totally have this under control, right?! :-)
• You really need to clean-up your language. Your brain BELIEVES words that you say. So when you say "I can't do it" - that sends a strong message to your brain. In fact, this is almost completely a mental battle. If you say "I can't do it!" or if you say "I am going to do it" - you are right. Where your brain goes, the body follows. Your words and your attitude are what will make your outlook better.
• As far as music goes, it sounds like you are somewhat accomplished, so surely you know better. You learn the piece VERY slowly - the form, the notes. That is phase 1. Then, you bring it up to tempo - that is phase 2. And you know phase 2 takes far less time than phase 1. Once you have the piece nailed, slowly - your only task is to practice endlessly to bring it up to speed. That's what you need to do in the next 12 days.
  
• Even if you do fail, that should be no loss. You failed while you TRIED something. Most people would be too afraid to even do that. I'd rather try and fail, then to be too afraid to even attempt it.
  

So my advice to you:

• "Fake it until you make it." Use positive language with yourself. It may seem silly, but there have been research studies about this - it makes a HUGE difference. Pretend like you got this; pretend you are confident; pretend that you know you'll make your deadline and that will change the chemistry in your brain.
  
• You know what you need to do - be methodical about it: learn the piece slow, then bring it up to speed. That's all you can do - if you are not up to full-speed by the show then it is what it is, you did all you could.
  
• Even if you fail (but don't spend any time thinking about this option) - it doesn't matter. You tried. Learn from it, cringe, and move on. At least you have the balls/ovaries to make an attempt.
  

Now stop reading reddit and go back to practicing!