What you describe does not suggest a lung problem, despite the 10 years of heavy smoking. Your age, presumably normal chest x-rays and breathing tests and your ability to exhale "fine" suggest that you do not have significantly diseased lungs. Some damage might be expected from the smoking but the key word is non-significant. If you have difficulty breathing-in, the problem may reside in your voice box or vocal cords. Yawning is seldom a sign of lung disease.
If our assumption that a breathing test was part of your doctor having "found nothing wrong", is mistaken, you should have spirometry done. It should include what is called an "inspiratory and expiratory flow-volume loop". That would provide the best measure of any kind of obstruction to breathing.
I'm surprised to see your question because I was just about to post something VERY similar. I am also 26 and a female. I am having constant shortness of breath. It's like I start to inhale but then I have to stretch to get enough air. sometimes it doesn't even feel like I'm getting a full breath. This started 2 years ago...and I'm ashamed to say I am still smoking. I have quit a couple of times. The condition will be very noticeable at times but then other times I hardly realize it. When I went to the doctor, she did a breath test and a chest x-ray. She said that everything is fine with me and that I should go on antidepressants because she believes my real problem is anxiety. The other thing i notice is that if I move my shoulders up and down one at a time I can hear my lungs making a noise(kind of like air flowing). I'm not sure if I'm over worrying and that is causing shortness of breath...or if my shortness of breath is causing me anxiety.
I'd also like to mention that I do get a lot of exercise including walking, biking, and tennis.
I should also note that I have no cough, and no pain that I can associate with this.