Sara,
On the bike I use for climbing I have a tripe 54/39/30 and can replace the 12-25 with an 11-28. I don't like the 11-28 for normal road riding because of the gear spacing. So depending on the ride I switch out cassettes. This bike is used for the long climbs, those lasting miles. My other bike is set up just like yours, I can climb with it as well but I can't do the long steep hills.
Other options for you would be to change the chain rings. Although I have not done it, you may be able to have your inner chain ring swapped out for a 30 or 32. You would probably need to a chain drop guard on the bike as well.
Now to what Stapfam said - climbing also depends a lot on your fitness level, both strength and weight. For me I know I don't climb comfortably until my weight is low and I have built up my strength. Since I live in the great North east I have to go through this pain every year as I work off the winter fat. I am currently training for a hill climb race, Only 8.5 miles but up 3500' of vertical. I go out and seek hills, small 1000' climbs at first build up each week until both my strength and mental will is back. So don't try to be a great climber overnight - it takes time. One other thing I have noticed, if you just gear down, you can reach a point were you are actually slower and more uncomfortable on a climb, climbing requires force on the chain and it's hard work - there is just no way around it.