Ir sb se pehle tu ap ye dar nikal dein height ka!!!!!

1- As Moon said, better get uterus checkup by a surgeon female doctor, it may be cause of infection which your consulted doctor could not spot/detect. jessa k ap ne kaha k 6inch insertion tak apko dard nahi hota, that means the infectious spot is beyond that.

2- as long as size is concerned, vagina can fit any size, tissues stretch ho jatay hien according to the penis girth. Plus it's not your first time trying this size, it has been years already. The problem could be that the inside of vagina is not lubed enough to smooth rub of penis. A similar problem had with my friend. It was solved by using lubes and cooperation.

3- The problem happens during one position only? try queen's position, sit on top of penis and take in as deep as you can bear, control will be in your hand.

All the best, hope above is useful.