You need proper evaluation It can be tumor or cyst Meet your dentist or physician as ap

You will need a clinical and may be radiographic examination of the swelling. As per the history provided it may be a benign swelling but 17 years is a long time, you will have to visit a specialist for the same.

Don't worry, as it is in same size from so many years, it is not a serious condition. But, obviously it needs diagnosis as early as possible.

Sir without clinical examination we can't comment and if possible please post pictures

Considering that you have diabetes, you should consult a dentist in person, immediately. Do not ignore it. Also your description of it being hard to touch is alarming. Best course of action will be to get a consultation and associated investigation procedures, after which a diagnosis will be reached and the problem and it's treatment, if required, will be explained to you, after which you take the necessary steps.