Hi,
At 300 it is high but not desperatetly high, you will be going hyperglycemic around that figure and heading in to elevated ketones. Above 350 and you need urgent medical intervention. I have been averaging over 400 daily and can tell you the sooner you act to reduce it the better. I have been extremely lucky that my ketones don't elevate much, but none the less I'm in the danger zone and unable to be medicated. Diabetes is painless until you leave it to long, then the symptoms start presenting, none of which are particulary nice to live with. Most of the bodies critical organs are affected by diabetes left unchecked, it becomes a domino effect.
I make it a rule in my house hold, no forbiden foods to be bought into it. That way I can't eat what I can't see and am too lazy to get off my butt to go get any of it. I make a shopping list of the acceptable foods and stick to it when shopping. A good start is food in moderation, cut out the inbetween snacks and stay away from the nasties. Drink water to fill the stomach between meals, it helps the kidneys do their job and filter out the excesses.
I found you can do anything when your health becomes threatened. Your body has given you a very clear warning to act or pay the price.
Good luck.
Cheers