So I have terrible dysphoria about my upper body, and sometimes I feel hopeless.

I want to know if HRT can change my upper body to a smaller and more petite frame? Thank you.

Bone structure will remain unchanged, the rest is up to how much muscle/fat tissue You have around those areas. I lost a couple of inches (from both sides) with regard to shoulder width and still keep losing muscles around ribcage. In terms of clothing, I went from L size for males (EU 50)to somewhere in between of S/M for female (EU 44) - or to put it simply, when I put on my former clothes, they look like I had taken someone else's stuff .

Oh, and when I started, I was desperate everytime I was looking into mirror and seeing those shoulders, upper body and neck... I wish I knew back then .

It will certainly help but may take some time. It really depends on how bulky or muscular you are and how broad your shoulders already are. if you're built like Schwarzenegger then you may struggle for a while. If your build is maybe closer to Woody Allen then maybe not so big a problem. It all boils down to what you're starting with - but even women who start out bulky find there is some positive change.

Well, I looked into Your introduction thread where You stated that You are 12, so I guess at that age You cannot really be that big or bulky... Your body probably has not even started to change Your bones according to male pattern, so if You can get on blockers it will be all fine for You!