Diabetes has many different etiologies.
While some forms can be avoided by simply watching diet, there is evidence that there is genetic predisposition and also a viral or chemical cause, in some cases.
Mammo provided excellent advice.
Do not be afraid of taking the metformin. If taken according to doctor's orders it has an excellent safety record and will be very helpful.
You may find, after a period of exercise and weight loss, you may be able to stop taking it.
While you can take steps to keep from developing Diabetes, if you have it in the family, you are pre-disposed to it. But being pre-diabetic also means there is a good chance you can turn this around with diet and exercise. Eat a low carb diet, avoiding the bad carbs like white flour, pasta, sweets. Read labels and educate yourself on what you can and can't eat. Low carb, no trans fats, lean meats, avoid white bread...anything made with white flour. Exercise is very important. Goodle "diabetic diet" and learn what to eat and what not to eat. People can and have been able to stop taking medication when they change their lifestyle to a healthier one. I'm sure you've been doing a lot to prevent this, but there is sugar lurking in foods you'd never expect it to be in. Education is key to reversing or controlling Diabetes. I wish you all the best and take care.