Looking here:
http://www.rxlist.com/tresiba-side-effects-drug-center.htm , I note itching and rash on the list of potential side effects, but not specifically thrush (although that can itch and have an associated rash where you have the thrush).
I think it's probably time to see your GP to see if he can give you something for the thrush. If you've already done that, then if he's not able to help, it might be time to ask to speak with someone who might be more specialised.
Alternatively, maybe whomever you see for your diabetes could consider you having a trial of a different insulin, as you seem to feel the change to Tresiba and thrush happened about the same time.
Having had a couple of times, over the years, you have my sympathy. Mine was never anything like yours, but uncomfortable nonetheless.
Good luck with it all.