Oh Jan, I am sorry you are feeling this way. As Morwenna says, there is nothing any of us can say to make you feel better.
Have you considered seeing your GP to see if there is anything he can give you to stop this anxiety? I'm sure you will get an appointment tomorrow if you push for one. Meanwhile, you need to take care of yourself. Sleeping and eating are essential, and especially fluids. Fresh air is good, too. Have a daily walk, listen to the birdsong, and see some friends.
I know you are having surgery at the same hospital I am using, and all I can say is you are in excellent hands. The staff are wonderful, all of them, and will look after you so well. You will be in and out of that theatre before you have time to think, and any pain you may have will be well managed, then home in no time. I didn't have any reconstruction, just a lumpectomy and a sentinel node removal, but can honestly say it was a breeze. I didn't need a single pain killer. The worst bit was waiting for the path results!
All the best to you, and take care. Sending you hugs, Linda xxx