You have done the most amazing job, despite the tough start you had.
I bet there's a story behind the story. How did things go with your baby's birth?
Babies "imprint" feeding in the first hours after birth. Under normal circumstances they are born alert and ready to suck - on whatever nature supplies them with. They don't know about "flat nipples". All babies will shape the nipple and breast tissue into a teat with their mouth. But if a baby is born affected by a difficult birth, or drugs taken in labour, then it becomes a much harder task. Likewise if mother and baby are separated during those early hours. As you have seen, patience and determination can overcome these problems.
Do you see yourself tandem feeding?