Ideas, mostly suggested by professionals (a Physical Therapist and a Speech Pathologist), for me, and not necessarily applicable to anyone else, but for what it's worth, here are my ideas:

Consult a physical therapist and do what they tell you.

Get a Theraband or a piece of stretchy cloth. Loop it over a door handle or fridge door--stand so that pulling the band doesn't continually open the fridge. Wrap the ends of the band around your hands and stand back until you feel a nice, hearty pull when you draw your hands toward yourself. Pretend you are rowing a boat, or pulling up on a dog's leash, or pulling stars down from the sky. Do that a few times every few hours--do NOT overdo.

There are lots of other exercises. Those muscles that pull your scapulae together must be teensy by now.

Slouching can also interfere with your breathing. Practice holding a dial-tone type Ahhhhhhhhh...as if the doctor just said, "Say ah." Do that a bout five times twice a day. Jot down how long you can make it nice and even without gasping, choking, or otherwise ruining the dial tone.

Take up singing. The above will help you sing better than you do now.

Best of luck, and you will be amazed how much better you will feel with some oxygen available.

Jaye