Perhaps you notice from a photo that your shoulders are rather rounded
They didn't get like that overnight, so don't expect to change them overnight!
If you 'try' to pull them back, you're just adding on another layer of tension. And whenever you're not thinking about them, they'll simply go back to how they were before. The muscles pulling them forward need to ease off a bit and find their proper tone.
It'll take time, but this is something you can change if you really want to.
Contact Kate if you're concerned about your shoulders!