Stick with it and the hill will get easier -- or you will get faster and it will be just as hard! Like others said, your best option is to reduce weight -- on your bike, tires, the gear you carry, and yourself. My commute route is very hilly. Although there are no hills as long or as steep as yours, my route is about 15 miles each way with repeated climbs and downhills. In the morning, most of the final 3.5 miles is uphill and it used to really blow my average speed. However, at some point this spring, all of the sudden my average speeds jumped about 1 mph, mainly due to faster climbing.

Does your bike have lightweight folding tires? That can make a huge difference in climbing. Also, keeping the amount of gear you carry at a minimum in something like a Carradice bag can really help. If you are not bike commuting every day, carry extra gear such as clothes, towels, food to work on the days you drive so you will have less to carry on cycling days.