Regarding scarring, when I had my Covid vaccination it was so far up my arm I didn't even actually have to take my top off. In fact, I was ready to take it off (super keen to be vaccinated!) - but the vaccinator stopped me and said I'd only need my shoulder out. I had a vest top on and a baggy shirt over the top, so it was easy enough to take out my shoulder and the top of my arm without showing my forearms (and the majority of my scarring). Do you have something baggy/easy to slip off your shoulder? It's so cold that I think a lot of people want to stay as dressed as possible!
They give you a choice of arm to be vaccinated in, but they do suggest having it in your non-writing arm as it can make your arm quite dead and sore for a couple of days.
I agree that a diazepam might be helpful to stay calm if you need it
I hope you've managed to get some sleep and hope it goes well today! My vaccinator was lovely - I can be quite nervous about needles, but she kept me calm and put up with me nervously rabbiting on about everything under the sun.