Sounds like maybe your reps are too high - you are just spinning your little legs out of fuel. If you don't have one - get a computer with a cadence monitor. Try and stay around 80 - 90 - if you find yourself often going over 100, you will use up your fuel too quickly abnd tire out your legs. The key to hills whether or are attacking them or gliding up them is maintaining a comfortable steady cadence. If you find yourself spinning too fast in order to approach a hill, your technique if off. Stay at 80 and adjust your gearing to compensate the load.