We know exactly how you are feeling. Please note that all of us have had this shock and came here for help... we've all found it and will now help you.
What you need to do at this moment is know that your cat is safe, diabetes is completely treatable and the 380 level is not really bad - yes, high and diabetic but some are diagnosed with much higher numbers.
Sometimes infection can raise the blood glucose levels so you do want to have that infection cleared up. In the meantime, that number does necessitate treatment of diabetes.
Can you tell us, was there a fructosamine test done to determine diabetes or did they just test the blood with a meter and get the 380? This is important. The fructosamine test determines whether or not the BG has been high for the past several weeks, not just today at the vet.
Take some time to read through the
FDMB Frequently Asked Questions <--- linky
This will tell you a lot about the disease and how it is treated. Read it twice and post here with any questions you have.
You do not have to buy food from the vet. A low carbohydrate food that does not contain grains can be purchased at your local grocery story or pet store. Canned/wet food of any kind (preferably low carb) is better than any dry. So you can check out some
food charts <-- linky to see what kind of low carb (under 10%) food fits into your budget.
You will be given some insulin. Please read the FAQ I linked above to learn about insulin and how you want to start at a low dose and only increase that slowly. To know how and when to increase the insulin, you want to work with your vet and the people of this forum that use that insulin (whatever one it may be- but tell your vet you do not want vetsulin/caninsulin). To monitor your cat's BG, you can buy an inexpensive human glucometer and strips from your local pharmacy and test the blood yourself on a regular basis (before each shot and some spot checks in between)... No need for regular vet visits.
There is a steep learning curve and it's all quite overwhelming at first... But you will be glad you tried it. It becomes routine and a special bond will develop between you and your kitty that you never imagined.
Read and ask and read and ask.... We've all been there and we all want to help... because we'll all had help.... And hopefully, in a few days, weeks or months, you'll be giving inspiring advice to newbies too.
Best of luck.