V.,
Are you anemic? If so, insufficient hemoglobin might be the issue...are you being treated for this? I hope that you are working with your doctor to get the help that you need in this regard.
In the meantime, here is the exercise...but also please follow up with your doctor!
The Divided Breath:
Imagine your lungs as having 3 parts.
The bottom third, the middle, and top third. On the inhale fill the bottom third first, then the middle and finally the top third.
Make sure that this is one continuous breath, but divided, with a slight pause in between each third. Hold the breath in for 3-5 seconds if possible.
Exhale in reverse. First blow out the top third, then the middle, and finally the bottom third of your lungs until all air is expelled.(remember the pauses)
Repeat several rounds for a toatal of about 10 minutes. Practicing this every day will not only give you control and structure to your breathing, it will strengthen and loosen the thoractic muscles, allowing better, more productive breathing, and relieving the chest tightness!
Best wishes and health
S.C.