I stop, wait until I can breathe, then ride again. If it's that steep, it takes a while to recover, even walking. Walking gives me a sense of defeat, recovering to ride again makes me feel like I conquered the hill.
If you have trouble getting started again, ride crosswise or slightly downhill until you get going.
It doesn't take very long, maybe only a minute, to lower your heatrate enough to continue on.
I usually get out of breath long before I feel lactic acid build up, on a hill.
Nanci
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"...I'm like the cycling version of the guy in Flowers for Algernon." Mike Magnuson