Hauoli, good for you, doing all this great exercise (brisk walking and yoga) which really does help expand your lungs, and thus helps your breathing!

I know it's difficult for many people to get in exercise daily, but I'd recommend people start slowly with a 30 minute walk each day. Finding the 'right' yoga class can be a challenge, since there is really little uniformity in yoga practices/teachers. I also recommend that if you can find a class that fits your needs and you feel 75% comfortable with, give it a sustained try (not just one or twice). Took me quite awhile to get with a yoga class/instructor that meets my needs and I look forward to going to. And many of us in the class do end up laughing at our yoga inadequacies!

I admire you trying different things to help with your lungs, as well as your perseverance!