There are a couple of reasons for not doing well on hills. The first is obvious - fitness and strength. If you're not fit you need to do repeats to gain the strength and fitness to climb.

The second reason isn't so obvious. Take it from me, the BIKE is a major factor in how well you do hills. I currently am riding a Trek 1100 aluminum. 3X crank with 32 in the back. I can grind it up some pretty steep hills but my HR and breathing go sky high if I try to push myself. I've been riding this bike for a few thousand miles and it has not changed from day one. I suck at hills on it.

My friend who just started riding a MONTH AGO can blast past me on any hill in the area on his new Orbea. And his gearing is 39X25. He's older than me, hasn't been riding, and still goes OTF on every hill. Why?? It's the bike. His Orbea is stiffer and is more efficient at transferring energy into motion than my Trek. Thus, his bike climbs better than my bike (all other things being equal).

So, perhaps you need to do some repeats, but you should also look at what you're riding. Equipment does play a big part in staying up with the group on hills.