I had this problem for many, many years. It started when I was in my teens but got really bad when I started getting full-blown anxiety attacks in my early 20's. (I am now in my early 40's.) People would even remark about my yawning, it was so noticeable. I always felt like I could not catch my breath, and when I would yawn or finally catch my breath I would feel OK for a minute, then the feeling would come again. It seemed uncontrollable. In my case, it was caused Chronic Hyperventilation Syndrome. Your blood gas levels get all out of balance because of all of the fast/deep/over breathing you do to try to yawn or get enough air. The yawning and trying to breathe actually make it worse, you end up exhaling too much carbon dioxide, and the feeling that you need to yawn or can't breathe just gets stronger. Vicious cycle.
There are relaxation methoRAB and breathing exercises you can use that will help. The book "Hope and Help for Your Nerves" really helped me. Sometimes just knowing the cause is enough to reduce the fear, which reduces the anxiety, which reduces the symptoms.
Good luck to you.
Angela