Reminds me of when I used to ride a BMX bicycle, and yes my memory goes back that far
If I stood up on the pedals I could lean the bike way over and still maintain a straight line. Something about my weight still being over the wheels I guess, I'm sure an engineer could explain far better than I.
Sitting straight up through turns is kind of similar but instead of the weight being over the pegs it is now over the seat which is a little off center of the wheels once the bike starts to lean. It can feel more comfortable sitting straight and remaining "on top of things" but the bike has to lean over further in order to turn.
It can be difficult for some, myself included, to trust the "controlled falling" feeling that leaning into turns can bring up. Body is closer to the road, thoughts of bike slipping out and crushing a leg underneath, no thank you I want to be sitting upright and ready to bail at any moment, but this is just your brain doing it's best to keep you out of harms way but actually throwing you under the truck instead.
Try hanging off the inside of the bike as oldfordguy suggests, your bike will remain more upright through the turns. Quite the opposite effect than sitting upright yes? Your tires gripping the road, your suspension pushing up and down, they don't care how your sitting in the seat.
If your sitting up then you're further up the sidewalls than if you lean into the turn. It's alright if your just flicking through some backwood road but if you're really pushing it through some turns then you should really try to retrain your brain. Yes the ground is a bit closer but your bike can handle it and has a lot, LOT more lean left in it than ya think.
Trust the bike to keep ya safe. It is a machine and engineered for the job