Well, I am petite, slim, and I think I am fit....
It seems as if my issue is power. I do climb up many tough grades, I always spin, and I still feel weak on some of them. I've gotta get my quads in shape, because my lungs rarely give out, once I regulate my breathing (I have asthma too, but mostly it's not a problem). I see myself falling back on a hill when others stand and sprint for a bit. I seem to lose all the strength in my legs right about then and have to spin slowly to get up.