Welcome to HCM
You are on right treatment of BDD. And for anxiety
during having intercourse is related to the trauma during your teenage years. Abuse during teen age cause deep mental trauma and it causes fixation of ones belief about intercourse, in a lot of people thinking about opposite sex changes, they don't able to make proper relationships. But my friend be have to move past the speed breakers of life, you can't control your past but you can change your present and future. Try to forget the bad time, its over now.
One method is to just recapitulate the bad time, I know it will induce severe anxiety in you but you have to face it, you will suffer with severe anxiety but then it will ultimately desensitize you with present anxiety.
Second is to talk with your boyfriend about the anxiety you are facing, you can hide the abuse part if you don't want to tell him. It will give confidence to you.
Third is to try CBT methods and deep breathing exercises, relaxation techniques
at home which will increase the tolerance.
Progressively you will recover from trauma.
Hope I cleared your doubt, if you are not satisfied you can ask again.