You have said that you are following the tighten mark on the pedal, I think you’ll find that the tighten marking is for the axle into the pedal, not the pedal into the crank, to my knowledge no pedal brand shows what way to tighten the pedal, just left and right.
My way of remembering how to loosen a pedal is to remember how to tighten, when you fit pedals to a bike you could hold the axle with a pedal spanner and pedal backwards to tighten on both sides, so hold the axle close to the crank arm and rotate backwards, that would be to tighten so it’s the other way to loosen, ps like someone else said, chuck a kettle of hot water on it just before, always helps.