I must be unique on Mumsnet, in that I liked my mum's reaction when I told her that I had my first period. Dm is the world's greatest drama queen, so I was nervous of telling her, but she just quietly gave me a huge hug, and congratulated me with a warm and genuine smile. It made me realise that this frightening change was actually OK. In an era when there was no sex ed, she had made sure over the preceding years that I knew what to expect and how to deal with it.
And if I'm unique, that's OK, too.
I do think that the threads of who we are and how we understand ourselves do unravel a bit with puberty. The analogy of not realising that there was a seam is perfect. How far they unravel, and how we reknit them depends on many factors. Resilience is definitely one of them.
Learning that it's OK to be you is hugely important. With the best will in the world, genuine love for me, and the desire to help me, my dm stood in the way of this by constantly telling me how I needed to be.
I have made a conscious effort not to replicate this. I do not tell my dc to tuck their tummies in and turn their toes out, for example. I do not show any dislike for certain aspects of my body. I do not burden them (at least, I try not to) with my hang-ups. I do not criticise their fashion choices. I expect the same standard of presentation from my boys and from my girls. I show interest in their ideas and choices, respect and not denigrate them. I try to disagree respectfully, not dismissively.
It seems to me that the first thing your child needs to see from you when they walk into the room, is that you are pleased to see them. Only once that is established should there be any reaction - positive or negative - to their presentation or behaviour.
This is even more import for girls, as society puts so much judgemental emphasis on their presentation and behaviour.
Of course there is more to resilience than being comfortable with the way you appear, it is just one factor.