Hello! I have been through your question. Related your concern I think the most probably diagnosis is skin candidiasis. You should know that candidiasis of the skin often causes a red, itchy rash and grow in warm, moist areas like you have between your breasts. It is known that in woman with large breast candidiasis is more common in this area. While the symptoms are often bothersome, they can usually be treated with improved hygiene and antifungal creams or powders. But in your case, that this situation last almost a year, I suggest to meet a dermatologist to do a culture skin test to define better the diagnosis and to start antifungal medication like clotrimazole cream,or miconazole cream except the good hygiene( wash and keep dry the skin). I hope my answer help you I wish you a quick recovery.