I have bad asthma and need to lose weight?

I have put on 2 stone over the past year and its getting me down i have tried to exercise but my asthma always get aggrivated so i have to stop. i have tried dieting but dont seem to be getting anywere. does anyone know of anyway to lose the weight without making my asthma worse i get out of breath so easily even by walkin round the corner or even just laughing at something. please help its getting me down