Just keep on trying. I have a couple hills around the corner from my house that have some spots that are 20% grade, with an average grade of 10%+. The steeper one is only a mile, but the first 1/4 mile climbs over 200'. The other one climbs to 1100' over about 2.5 miles. It took me several attempts (and a large rear sprocket), but I finally conquered both.
Don't give up. Get a cassette with a wider range of gears if you have to.