Hi Tango,
You should be able to get your HGV licence back but you will need to explain to DVLA that you were misdiagnosed and you will probably need a letter from your GP supporting your request.
As Sarah says, first you need to be absolutely certain whether you really are type 1 or 2. If it hasn't already been explained to you, it would be worth your asking your GP what were the grounds for him thinking originally that you were type 1 and why does he now think you are really type 2?
You should also bear in mind that GPs usually do not have a detailed knowledge of diabetes, its causes or treatments and if you are in any doubt (which I certainly would be in your case) you should ask your GP to refer you to a consultant. Your GP cannot refuse this, and may in any case be relieved to have the decision taken out of his hands as he certainly seems to be floundering!