Hi.
Thanks for your query and an elucidate history.
This looks to be a problem called 
balanoposthitis and associated 
dermatitis of the 
scrotum ( testicles ). 
The effective treatment is as follows:
Clean the area with plain water 2 to 4 times a day and and keep it dry with a pure cotton cloth or a dryer.
you may apply liquid 
Povidone Iodine locally or a 
Metronidazole cream- nothing else at all. We have seen no results with any other solution, cream or ointment. So do not try anything else; if you are using any, please stop it.
A course of antibiotic ( you are already under a good antibiotic) and metronidazole orally for 5 to 7 days and a single dose of oral anti-fungal tablet should help you to get a cure. 
Please get your Blood Sugar checked.
If you are sexually active , your partner too needs to take the same treatment. 
I hope this answer helps you to get a proper solution, please feel free to ask further if you need any additional information
The treatment has to be taken by both of you and simultaneously