Hi Jeremy,
I know this is stressful for you, but you need to give yourself a warm reassuring pat on the back because you really have learned an awful lot in a very short space of time. And that 'learning curve' that you're one will soon level out somewhat and you'll suddenly find yourself thinking, "Hang on, I know how to do this...!"
Yes, testing is stressful at first. I never, ever, ever thought that my cat would tolerate testing. But even as I type this he's sprawled out next to my computer keyboard just hoping that I test him - because he gets treats when I test him and my desk is our 'testing spot'! :lol:
Yes, if you can, always - at the very least - get a test before you give insulin (what's called a 'pre-shot' number). And the general advice here is that you don't give a shot if your cat's BG is 11(200) or below. The next most important test to get is one maybe five (or six in some cats) hours later when your cat's BG will be at it's lowest. Any shots in between, and especially on the way DOWN will be hugely helpful to you.
Regarding high numbers, my cat Bertie was the King of them! Initially he didn't respond to insulin at all, and I felt absolutely sure that he was doomed. (And I'm Scottish born and nobody does 'doomed' like the Scots! ;-) ) However, here we are, nearly six years after his diabetes diagnosis and he looks way better now than he did back then!!!
Chin up, Jeremy; you're going to do just fine! But PLEASE be patient with yourself...