I’m so sorry boys were awful to you when you were growing up. This sort of behaviour has such a profound, longlasting, negative impact.
I was lucky not to have been bullied about my flatchestedness, but even so, I found it very hard when I was young. I felt left behind, and also socially and sexually irrelevant at a time when I was utterly boy crazy.
There was also an ominous feeling that a crucial process had failed to initiate in my body’s development, that may have other implications down the line. I was late getting my period as well - it was a huge relief and reassurance when that finally arrived.
By my 20s Kate Moss was famous, and that helped enormously - I finally felt like my body was of a format that people could understand and accept as female and adult and healthy.
In my 30s, I experienced infertility and multiple miscarriages and a stillbirth, and it was hard for me not to go back to that foundational feeling of being insufficiently or dysfunctionally female. I did eventually have a healthy baby, and once breastfeeding was done I finally felt able to just not care anymore about being flatchested. My boobs are retired.
Perimenopause has helped as well, and finally being free of the idea that the only point of me as a woman is as a sex object or a caregiver. I hadn’t even realized I thought that way until I very suddenly didn’t at all. It was like waking up from a terrible fever dream.
I’ve had lumps in my breasts a couple of times (benign cysts fortunately) and noticed them immediately because there’s so little tissue there, so that’s a bonus.
I hope you find peace, OP.