Oh oh! I know this!

I'm going through something similar right now. All my life I have been slouching. Not because of big boobs, but because I was taller than I wanted to be (the classic "slouching will make me seem less tall" everyone tall knows too well) and I was depressed and had a lot of anxiety all my life (until recently). So in addition to slouching, I walked around with my shoulders raised up for 30 years or so.

Not so long ago I decided to get in shape, so I got a personal trainer. She pointed out my bad posture and high shoulders and said "we need to get those down and raise you up, girl".

So it began. I started streightening my back as much as I could and poke my chest out. And damn was it uncomfortable!

My PT told me that all the years of bad posture and high shoulders ha made the muscles and tedons around my shoulders and chest too short for me to stand up. So they needed to be stretched into the proper length.

During the first weeks, whenever I stood up straight, it felt like my ribs was coming out of my skin. And the shoulders were just as bad, it felt like I was hanging off a clothes hanger, if you follow me. Not a pleasant feeling at all.

Now, after three weeks, I have a better posture and I am able to relax more with my shoulders as well. It's still quite a workout to sit/stand with my back straight, but my PT tells me that will improve as long as I keep trying. I think working out really helps my progress.

I stretch a lot (making sure my body is warm first), that really helps too. If I were you I'd look into getting a personal trainer who can show you exactly how to do this right.

Hope this was helpful!