OP, yes it's THE obstacle to you making progress, thefact you can't even mention your 'main issue' to a therapist.
Maybe it would help if you consider that so many women hate a part of their figure or face, you'd be surprised! Lots of teenage girls get teased about something - short legs, big nose, flat chest, you nane it. Many aer very insecure or even go for plastic surgery when they can. But so many do live with it, and uit's not because all their lovers praise them, some do and some criticise.
But people aer normally ok to open up in therapy and maybe it would hwelp yo uto think that therapists (especially female ones) have heard it ALL when it comes to body shape flaws or even body dismorphya, and a large bum is one of the COMMON insecurity issues.
I know your mother has done a number on you - it should be criminal to be so psychologically abusive to a child! But could yo utry and detach, and sort of observe your sesssion with therapist as if from above, pretend you are waatching another woman confessing about it and going through treatment - would it seem so bad if you imagine another person with your shape confesing to hating it and being stuck by the comments?
It's not just physical flaws that bother people either, plenty think they have unloveable personalities - in your case it's not that, and I can tell it's harder to ignore your own personality as you can't disguise or change that at all when in an LTR.
I'd also advise not to give on dating at 50 - older men aer often wiser and more tolerant and I know you aer worried about being fetishised by any who like your big hips, but I think if anyone wants to meet you (online for example) why not go and see - it's often that men flirt or show their attarction by complimenting you on something specific, but it's usually not the whole man - you may find it develops into him liking your personality which does take a while longer in any case. Don't immediately dismiss anyone who starts their attention with the physical.
Going back to therapists, try someonewho specialises in body dismorphya, even if it's not exactly what yo uhave, they d be so used to hearing the same concerns that you may feel more relaxed and also they may be more useful to you with focus on the physical.