It is true that diabetes can delay the treatment of fungal infections and you may need to use topical and oral antifungals for a long time. To confirm whether this is a fungal infection or not, you should get a KOH examination of the skin scrapings of the lesion done.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.