Breathing problems

for a year now it has felt like i couldn't breath, i would have to take a deep heavy breath to make myself feel better. but recently it has gotten to the point where my body will start to jerk and go on alarm and no matter what i do i feel like i cannot breath. it has become so bad that i am afraid of what might happen.i have been to the doctor many times, but a first they didn't know what it was, now i am being told it is stress. i am 15, 5'7", and weigh 102 lbs. what can i do to make it better?????