Erm, yes.
You realize that gravitational acceleration and motion aren't the same thing, right? When you're being pushed forward at 1 g (such as by a car or an amusement park ride) you're simultaneously being pulled toward the center of the Earth at 1 g, for a net total force on your body of 1.4 g at 45 degrees down and back. The horizontal component of that acceleration is exactly the same as what you experience lying flat on your back - you're just perceiving a greater acceleration since you're not used to experiencing 1 g forward accleration in a car.
You're muddling force and acceleration. And missing some forces from your example. And confusing the directions the forces act in.
Allowing for the common simplification that the surface of the earth is an inertial reference frame (which I realise in fact it is not, but this is not an orbital mechanics question so let's go with it) then while gravity exerts a downward force on you, equally the floor (or whatever you're standing on) exerts an equal upward force on you. The net result is 0 acceleration [in the reference frame of the surface of the earth]. The same is true of your eyeball - it is pulled down by gravity, and pushed up by your eye socket (which is pushed up by the rest of your head, and then your neck, and skeleton, and socks, and shoes etc).
In your example of the accelerating car, there are 3 forces - the car tyres pushing horizontally against the road, gravity pulling downwards, and the road pushing back upwards to match. The net of these is a single force in the direction of travel. Also your directions are wrong - the force due to acceleration of the vehicle points forwards, not backwards. If it pointed backwards you'd move backwards, and if it pointed downwards at 45 degrees that's where you'd go.
The reason you perceive that you are being pulled back into your seat when in an accelerating car is precisely because of the way the force is applied to you through the seat. The seat pushes forwards, which pushes the rear of your body forwards, which due to inertia compresses your body so that on average your body moves nearer to the seat, which you perceive inside the cabin to be "moving backwards".
Now you talked about "severity" of acceleration. "severity" is not an SI unit, so I assumed you were really talking about how the forces acting between components of your body are perceived (i.e. how you "feel" them).
As I already said, if you accelerate 1g vertically due to gravity [again, static inertial reference frame at earth's surface] such as when falling off a cliff then there is no "severity" because the force that acts to accelerate you towards the planet acts uniformly and equally across your whole body - the force between your eyeball and cranium is 0; between your head and neck is 0, between your liver and intestines i 0, etc etc. You don't "feel" anything at all.
However if you sit in a car and accelerate at 1g then you do so because the seat pushes your torso, which tends to leave your head, limbs, organs, and bodily fluids slightly behind. Which feels quite severe.
Put it another way, if you believe that two forces each cable of accelerating your body at 1g, but applied via different methods, are equally "severe", then presumably you'd be OK with someone tying your little finger to the back of a P85D and doing an insane launch? No part of your body would ever accelerate at more than 1g so it would be fine, right?
