Google - What am I suppose to do if one of the athletic people in my school makes me feel horrible?

You have to look for many things in this case:
First, you are becoming victim of inferiority complex because that one person whom you are thinking 'ATHLETIC', may not be that much real better performer.. So, jump in and race with him/her.. May be you will become better at running.
Second, Probably, you are feeling tired or less energetic as compared to him/her then take time and build up yourself. Time is best Medicine.
Third, Why you are thinking with such intensity about him/her, May be there will be many other people more athletic and energetic than to that particular person.
As far as my opinion concerns, then give yourself some time, train yourself mentally and physically.. Plus, it will be better to put concentration on positive sides neither on negative ones.