Aw, poor you, it's only nerves getting to you - have a good blow and dry those tears.
Although you may be a GBPFW YANBU to draft a Will at this particuar time, and think of the hours of fun you'll have threatening to change it throught the course of your (I suspect) very long life.
You can write a Will on any old piece of paper; remember that your executor/s can be beneficiaries, but in order for the Will to be legal your signature must be witnessed by two other parties who are not beneficiaries.
If you draft a Will tonight you can take it with you tomorrow and ask 2 of the practice/nursing staff to act as witnesses to your signature - I expect it's a common request.
If you find it difficult to sleep after you've divided up your worldly goods, why not plan your funeral? That should cheer you up. There's lots of good music available on the (I don't get) Opera thread, and I suspect there's a few poems on the similar Poetry thread that would suit the occasion.
Imagine how proud you'll be if you pop your clogs any time in the next few weeks and your dear newly bereaved DNBs' praise your foresight. But I doubt you'll be RingIP for long - if I had a relative with that level of prescience I'd hotfoot it round to the local Spiritualist Church asap to make contact and get the winning lottery numbers.
I have no doubt you'll come through your session in the dreaded chair without incident but if, in the so highly unlikely event that the odds are incalculable, you don't please do feel free to drop into mine with those numbers and move a few ornaments around before you go into the light.
With very best wishes for your continued dental and physical good health. You're going to be just fine, and I hope you'll be back here within 24 hours to tell us all about it as I lurve happy endings.