I also have "posture" issues, but I keep realizing they are largely about old feelings of hiding / protecting my body.
a lot of that is gender related, but not all.
I do find that as I transition more (close to 3 yrs this time for living as male 24/7) I am able to have times, hours, parts of days, where my posture is naturally much better. Forcing the changes didn't help. FOR Me, which may not be true for you, I have gotten lots of good help by going to trans sensitive chiropractors and bodyworkers (I used to be a massage therapist and yogic breath worker, yeah, with bad posture

)
Anyways, I truly think that the spine often holds trauma and tension for many reasons. Especially the pattern of flight or fight, which can happen due to stress around gender, life, car accidents, abuse experienced as a child or an adult, that pattern stays in the spine, and is very much part of why I haven't had great posture. The more I heal, and grieve, and release old traumas from cellular memory, and, perhaps most importantly, feel loved and admired for my real self, the more I stand up straighter at least a lot of the time.
I know there is so much to pay for, and it is an insanely vulnerable thing to get massage / bodywork during early transition (really, I know, I used to teach all those things mentioned, and still ran into deep fear / terror going for some of my sessions.....it was really hard to let my changing, vulnerable body be touched sometimes).
I wish you all the best. I think the more you can relax, and soften in your body, the more extra beautiful you will become. I think you are pretty already. Best of luck.
Joseph