Hi Roger,
No, you are not right. If it was that simple there would be no diabetics. It is not your obesity that is making you diabetic. It is your body's inability to metabolise your blood sugar. Being obese doesn't help that problem, but it isn't the only cause of it.
When a normal (i.e. non-diabetic) person eats something sugary it sends their blood sugars rocketing, the same as it does with yours. The difference between them and you is that their bodies are able to convert that sugar into energy which is stored in the body's cells.
You may be able to get your sugars down to a normal level, but that doesn't make your metabolism normal. If you eat something with a high sugar content then your body will still struggle to convert that sugar into energy.