More info needed:
That 20.5 gear inch number seems low enough for most any paved hill. It's around 24 front, 32 rear, for example. You should be able to climb quite steep hills with it. The rear cogs have tooth counts stamped on each cog, or you could count the teeth. What gear numbers do you have in your low gear?
How long and how steep are those hills? Can you keep a slow speed, around 3.5 to 4.5 mph on the steep parts?
From your original post, about your muscles needing a rest in the middle of the hill, I assumed you were grinding up the hill in a very low gear, at a very slow cadence and pushing hard on the pedals. As opposed to using the easiest gear, keeping the speed as slow as possible while still being able to steer and balance. But then you mentioned the very low gearing.