I've got a hill at the end of my commute that depending on which side I approach from either averages around 12% and peaks at 20% or averages around 8% and peaks at 14%. The steep side is 1/4 mile long, the gentler side is about 1/2 mile long. Both are exhausting, but in different ways. I mention this as background information.

My experience is that I need a low enough gear to be able to maintain a reasonable cadence, but below a certain point (the 22x32 gear on my mountain bike, for instance) I feel like I'm just not getting anywhere in spite of putting in a lot of effort. You just need to find the gear that's comfortable for you. Which gear that is will likely change over time.

As for more concrete advice.... First, FBinNY is spot on about training. You don't need to use a hill that steep to train for a hill that steep, and doing so would probably be less effective. You need to build the strength somewhere else. Second, and I personally think this is more important, don't be ashamed to get off and walk. If you can't make it, you can't make it. So what? Maybe you can make a goal of getting up the hill, but until then it's not a big deal. I often walk up the hill at the end of my commute, depending on my mood. I can make it up the hill, but a lot of days I just don't feel like working that hard. It's a choice.