Hi Popcorn - one fasting glucose test will not be sufficient to diagnose diabetes. Many contributors who diet carefully to control their condition will get a fasting reading below 5.9. Try a reading two hours after a meal including normal carbohydrates. If it goes above 8 you could be diabetic.
It seems your primary concern should be your weight, & a drastic, unsupervised 500 cal diet is not sustainable, especially combined with exercise. The sort of diet I would suggest is say a 1500 cal diet with your calories from protein, & fats including the animal fats & olive or or rapeseed oil, especially nuts, which are a good combination of proteins & fats. (Not peanuts & cashews.)
The surprising truth is that fats don't make us fat - carbs do the damage. Fats are slow digesting & therefore keep us full longer, so we do not feel hungry before the next meal. The body soon learns to use fats as its energy source.
Sadly you are unlikely to get useful advice from a dietitian - the standard "health diet" recommendations include about 40% starchy carbs - which are all the foods that metabolise to blood glucose & excess is stored as fat. Exercising will preferentially use carbs rather than fat reserves. My dietitian has now changed her advice based on my experience - she even invited me onto a diabetes course as an example to others. [I'm 73, 12 years diabetic, suffered crippling complications following her diet, & have cleared all complications with a low carb high fat diet. Tennis this evening - 5 sets last Tuesday.]
Look around the forum - you will see that almost everyone has benefited from carb control.