I feel for you. Our minds create such possibility in us to live a wonderful life. Sadly it seems it can also cripple us too. Now I am you size, I guess, no professional at psychology and have no easy answer, but to some extent experience similar triggered shame, except it is about cumming. The more someone expects and begs me to cum, even fucking raw, the more difficult it becomes. Seems I have shame about cumming - unless I am with other what I could call 'shameless' people also bare fucking and dumping loads in mouths and arses - analgous with your ease with similarly-sized people but not alone. It seems it is fear of disapproval (ridicule) which triggers the shame / self-loathing you have learned to replay when these conditions appear. The question is how to disconnect that learned trigger. I wish I knew for both our sakes.