This type of situation is indeed enraging and all too common. Vent away - many of us have been there! Sadly the world is full of idiots, many of whom work in the medical profession, many of those not realising how little they know and are not motivated to find out more.
I manipulate it to use to my advantage, as I find I can divert the power of my temper to doing the best I can for my diabetes just to spite 'em. I also take care not to show how I feel because that way I retain the moral high ground. A most unexpected spin-off was my latest "phone appointment to discuss my blood test" where Nursie was so nice I thought she'd had a personality transplant. So it is actually worth while in a deranged kind of way.