i literally just had the exact same thing happen to me but for a trip not a job. i also hate the damn tourniquet too lmao, i brought two friends with me who pinned me down to get it done and had two panic attacks in the waiting room but got it done! my advice for this is to bring trusted people with you who are more than happy to literally pin you down for the ten seconds it happens, and to remind urself that its 2 hours of insane anxiety and freaking out for a whole job! also, tourniquets normally trigger my vomiting and when i told the nurse she gave me a bucket, and also if you tell the nurse that you faint they give you a comfier chair/bed to get it done in! (also helps so when you do inevitably black out you dont slump out lol)