| If I were you I'd transfer from PADI to BSAC. BSAC has a full system designed to incorporate divers with diabetes, including specialised medical referees.
I can sympathise with instructors being nervous because diving is one of those sports which you're 'not supposed to do' with diabetes - it's one of the most dangerous conditions to dive with. However, you're already aware that this isn't the case. The best you can really do is present your certificate of medical fitness to dive.
As for doctors - utimately it's not 'your' doctor that can tell you whether you can dive or not. You should consult a doctor that specialises in sports medicals. Most ordinary doctors will know nothing of what's actually involved in diving, including the fact that it's one of the least demanding sports in terms of energy expenditure. The whole point of diving is to try and minimize your movements in order to minimise your breathing. | 
