Short breath my whole life, not asthma, iron levels normal

Hey guys. I'm a 20 year old female. My whole life (since i can remember, from at least age 8) I have never been able to run, or play sports, or do anything remotely physical that requires effort, as i get very exhausted and find it very hard to breath. The only think I can do is power walk on a treadmill, but I even have a lot of trouble with that if i push myself slightly. After activity, it will take about 10 mins to settle down and stop shaking and breathing properly. I can get light headed and very tight in the chest. I have been told by 3 doctors I am not asthmatic, I had low iron for a while there that can make you feel weak and out of breath... but that is well under control now. I eat healthy, drink lots of water and tend to stay away from take away foods. I have been to GPs, and all they do is give me inhalers, which don't work. I am not overweight, I weigh 53kgs and are 167cms tall - the average for my age. I do not have health problems and have never smoked.