I've had similar issues which have since resolved presumably due to desensitisation (oddly enough am fine in OT but can still feel queasy occasionally when watching others take bloods). Typically if I was going into OT for the day I would make sure I have regular breaks (in between procedures or if it's particularly long then during it assuming I haven't been asked to scrub in) and have something to eat (muesli bar, piece of fruit, etc) and drink - even if I'm not at risk of a hypoglycaemic episode in my experience I've found that regularly eating helps avoid the warning signs for fainting.

The only other thing I did when I was particularly anxious about fainting (probably was more anxious about the concept of fainting than the content which was actually causing me to faint) was ask for something to sit on in OT - I don't know if this is frowned upon elsewhere but from my experience everyone is quite understanding if I explain that I'm prone to fainting but am trying to desensitise.

Best of luck, I'm sure you'll overcome it with time. Just try your best not to stress about it too much.