l play soccer, but everything gets me out of breath, and l am fit, and l can only breath through one nostril

In through the nostrils.
Exhale through the mouth.
Deep and slow.

starlet- The best way to breathe is continuously in & out ;-)
Deeper breaths r better for a greater oxygen intake & expiration of carbon dioxide build-up during soccer play. Nose bbreathing is better than mouth-breathing because it does less drying of the mouth & actually adds some moisture to dry air u breath in. If u can satisfy ur body needs by breathing through one nostril, that is OK. If not, check with an ENT (ear, nose & throat doctor) to try & open up the second nostril to breathe in more air. As for getting out of breath, that is common among active sports players. Ask ur teammates. Maybe some exercises could be suggested by ur coach. Good luck!

If you have to think about it, you're not doing it right!

Seriously, have your doctor do a pulmonary function test you might have low grade asthma and not even know it. This is very treatable and can greatly improve your quality of life. Also if you've got a full-time nasal blockage that never clears you might have nasal polyps. Also very treatable.