You sound just like me a year ago! In fact it's exactly one year since I first posted that I was "newly diagnosed and panicking" (Feeling pretty great now...quite a bit better than before the diagnosis, actually! :)). It's such a worrying time and one minute you feel in control and the next you're all over the place. As the others have said, the pre-op is nothing to be worried about and as for further treatment, it probably won't be needed and if it is, you'll deal with it. I can see you have lots of support on this forum so just keep posting when you panic, we will be here and very soon you will be far enough along the path to feel much more in control.
As 365days says, take one step at a time. Focus on getting yourself ready for the surgery. Make sure you are eating properly (I had to resort to Complan because the anxiety played havoc with my appetite) as a body well-supplied with nutrients will cope with the surgery and healing better. Exercise or meditation (download a phone app and give it a try) should help with the panicking. If you find yourself getting into a state about "whatifs", notice that feeling and then bring your attention back to the moment. I did a lot of crochet in the lead-up to my op and still do - very soothing when I was flipping out. Keep yourself busy organising your home if you have the energy to make it easy for you when you come out of hospital. Tidy underwear drawers and easy food made my post-op life easier! If you don't have the energy, just be kind to yourself - you deserve it.
Best of luck with everything!