Finding it difficult to breathe?

So basically, I'm 16 years old and I've found it hard to breathe for many years now and every time I visit the doctors they tell me my health including my lungs are perfectly fine.. I don't understand how it could be fine when all I do is struggle to breathe! The first few years I discovered that I couldn't breathe properly was from my right side which didn't bother me much because at least it wasn't my left right?! Otherwise I probably would have thought it had something to do with my chest.. But now, a few years later, it's happened! I can't breathe properly from my left side, and I'm pretty sure it's to do with my lungs and its killing me :'( I practically have to yawn or sigh to get a full breath with every breath I take! Sometimes I have these shooting pains from the left side of my chest when I breathe, sometimes from my back.. And oh yeah, I just realised, it mostly happens during the winter, or towards the ending of the year and currently it is winter. By the way, I do not have asthma and I'm not overweighted or anything so those things cannot be the reason.. Please help me discover what this is! All your opinions and advice would be great help! Thank you.