Most Helpful Guy
Losing weight will of course reduce the size of your back and shoulders, but you can't change your skeleton. If you just have naturally broad shoulders, like many women with Amerindian heritage for example, there's zilch you can do about it. Look at the bright side, you're much stronger in the upper body than most women, that can be an advantage in life.
Most guys aren't going to find you unattractive just because of this. Don't worry so much about it!
And don't waste time hating what you can't change!
Now some women dress in ways to minimize their broad shoulders, but imo that just looks silly.