you are putting yourself at more risk of a bad incident by unclipping on downhills, your feet are at risk of slipping off the pedals if you hit bumps and you are taking away the most important part of bike control--being able to stand, having pedals parallel to the ground and let the bike move around if it needs to--and to repeat, with the great risk of not having your feet properly attached to pedals, one slip and you could be in a whole mess of trouble.

I fear nothing I write will convince you of this, and from your comments you are not comfortable on a bike that moves around under you, but sincerely, unclipping on downhills and not getting your bum off the seat to allow the bike to move around under you is putting you at danger of worse than what you imagine.
I also realize that if you are uncomfortable with this stuff at your age, it isnt going to change, but at least try staying clipped in, getting your behind off the seat and get a bit of a feel for the bike moving under you. Even try some little tests of locking the rear wheel on a slight dirt downhill at slowish speeds while out of the seat but clipped in, just to get an idea of the feel.

My comments are sincere here, it really does make me cringe to think of you unclipping on downhills. Think of it this way, even riding on the road in the city, there are situations like being surprised by a pothole or an object on the road where the absolute safest thing to do is to get off the seat, and lift the front a bit and just let the bike bash around under you--being clipped in is just so much safer because while all this is happening, your feet/foot are not going to slip off the pedals which can lead to all kinds of disaster.
Being solidly braced/attached at the pedals, off the seat and a good grip on the bars is the best position to deal with an uneven surface or obstacle, as well as any traction issues going on.

"Putting a foot down" isn't really something you want to do, unless of course you are at the speed to about to stop.

I realize you didnt really specify at what speeds you do this and in what conditions, but my wife does the same thing and it makes me cringe when she does it also.
cheers