No one is ever going to question the validity of your claims, hun. You don't need to add "I am serious" we understand that you are. But you also need to be careful about self-medicating, if that is a route you decide to take. I know its a much wider-spread concept around the world, but it is very dangerous and difficult to get the results you want as far as I know(im not on HRT yet).
Susans Palace doesn't condone self-medication, and its against the ToS to discuss dosages for this reason.
I am 32. I was born with 'broad shoulders' according to my mother, after I told her how troubled I was by it she decided to explain how I had to be turned in child-birth to get out because of my should-length. yeah, thanks mom...
Women can have broad shoulders, there are fashion choices one can make to draw the eye away or cover the fact, too. Once you are properly on HRT, YMMV(your mileage may vary; meaning different results for everyone) but the ligaments between bones can change in size and flexibility which can lead to a slight reduction in size.
we dont fully grow until our mid 20s, I think I heard that somewhere. So if you do so before then, you have the chance to adjust bone structure somewhat, too, with HRT. Also, its said that each 7 years every cell in our body is completely replaced, so the earlier/longer you're on HRT, the better!
That being said. If your mother is adamantly against it, this could be because she doesn't understand it or she thinks its a 'phase' this is pretty common. There are things you can do right now without HRT to help feminize yourself, and perhaps when she sees this more, it may convince her to have you start HRT sooner.
I don't know the details of your particular situation or what its like for transgender folk in Serbia, but I hope this info helps you in some way. Take care of yourself and good luck!
Much love,
Eryn