Can't take a full breath keeping an upright posture.

Hello, I've noticed this a while back but never given it much thought, now I'm trying to have a better posture and it's worrying.

Every time I try to sit straight or stand straight it feels difficult to take a full breath from my stomach (I've heard that's a better way of breathing compared to chest)

Another thing is that my heart rate is about 90 BPM sitting up straight and 60-70 BPM sitting slouched.

I do have some pectus excavatum, had nuss but there's still a dip remaining. I first thought that must be the issue but I googled and a lot of people seem to have this breathing issue without pectus.

Is my body just used to slouching and has to adapt to good posture? Do I need to breathe different? Strengthen something?

Any help would be massively appreciated. ❤️