Hi Ellen,
The ones I mentioned are by no means a complete list. I don't know what strength of metformin you are on, but it might be possible to move up to a higher strength, and/or to a stronger sulphonylurea, perhaps Amaryl.
Sounds like your GP does know what she is doing (unlike many who just don't understand diabetes at all) and is trying to do her best for you, but you don't seem to be doing much to help her!
You mentioned that you don't watch your diet, but that is the biggest thing that you can do to reduce your blood sugars. You can make a huge difference to your BS readings simply by reducing the anount of carbs you eat. This doesn't have to be drastic - for example if you have cereal for breakfast (typically 35+ carbs) then occasionally have a boiled egg with a slice of wholemeal toast (around 15 carbs) or a bowl of porridge (25 carbs). If you have a sandwich for lunch, then have a salad instead. If you like pasta for a main meal then use a low-carb variety instead (Dreamfields is currently the only one available in the UK). You mentioned you have a sweet tooth - whatever you do, don't try diabetic sweets. They don't contain sugar but the substitutes they use have exactly the same carbohydrates as sugar! Just cut down even if you feel you can't give them up.
You also mentioned weight. If you are overweight then did you know that a 10% reduction in your body weight will reduce your HBA1C by a whole percentage point. You can bring your 8 level down to 7 without needing any additional medication to do it.
I agree nobody wants to have to inject, but you are in a position to ensure you don't have to. There's so much that you can do to help yourself, but you have to take the initiative. You said insulin injections will be inevitable - but only because you are making it so. And that is the start of a downward spiral - insulin encourages weight gain - weight gain increases insulin resistance - more insulin resistance needs more insulin - more insulin means more weight gain - and so on.
Your future is entirely in your own hands and its not too late for you to stop the spiral before it starts.