Riding uphill provides more natural resistance, which in-turn recruits more muscle to turn over the pedals. It’s not a strength “problem” it’s just a muscle recruitment problem. The best way to learn is to do long rides with a focus on more constant pressure. Maybe you ride at 200 watts on the flats and try to hold the same wattage going downhill as well. Eventually your ability to recruit more muscle in a variety of terrains will become easier though it may never feel 1:1. You can get pretty close though with enough time in the saddle.
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