Eliza, I think it's perfectly normal to be afraid of this surgery. I know I was very nervous, and I had had the comfort going in of knowing that the trial worked for me, which is unfortunately a benefit you don't have and I think that's also adding to your fears. When you suffer through chronic pain, any new treatment that you try it is so hard not to get your hopes up too high, to hope for a better future, while also preparing yourself for the possibility that it could not work. Also, it is scary to have something implanted inside of you. The thought of it, because it is not something that naturally belongs in our bodies. I hope I'm not sounding like a downer here because that's not how I mean it, but if I had to guess, these are some of the feelings you're going through, and I want you to know you're not alone and what you're feeling is normal.
A friend told me before my surgery that what ever happens, I am very brave for trying it. Those words really stuck with me and helped get me through my nerves. You too are very brave for trying an SCS, especially with how difficult surgeries are for you. Know and remember that

Also remember that everyone here is supporting you and wishing you the best, and we're here to listen whenever you need to talk or vent.
Finally I'll leave you with one of my favorite Bible verses: "Trust in the Lord with all your heart. Lean not on your own understanding. In all your ways acknowledge Him and He will direct your paths."