Hey, I am not an expert but I don’t think you should feel your fighting the kart, and you shouldn’t feel your being thrown around either. Bruising is normal after a weekend of hard practice or racing, especially on the hips, so that is not a good indicator of set up.
1) Your seat should fit tight, but too tight can be a problem. I find that it should be tight enough where I can barely slide my hand between my ribs and the seat once I am seated, but not too tight where I can’t fit anything between the seat and myself.
2) Steering wheel/ Seat placement is extremely important. If you are too far or too close to your wheel you cannot properly control your posture in the kart. If your seat is angled to far forward or too far backward (swept) it can make more difficult to maintain posture. Try playing around with your placement in the kart, comfort/functionality is huge.
3) Core is a huge part of karting, I am not saying your core is weak but you shouldn’t have to worry about pressing your thighs against the seat.
4) You should be able to move your entire upperbody forward or backward or left or right or neutral or a combination of these in any corner. If you can’t; there’s an issue(kart setup or physical conditioning). Sometimes in highspeed corners I’ll lean inwards while in the corner to take grib away, and sometimes(especially in the rain) I’ll lean outwards in the corner to add grib/weight on the outside of the kart to allow it to turn better. All this depends on set-up, height, leverage on the kart, grip levels, etc, so it just an example of the importance the driver posture has on the kart performance.
5) It is important to anticipate what each corner or input on the kart is going to do to your body and adjust accordingly. Stay ahead of the go kart and try not to be surprised by what it is going to do to your body position by then it is probably too late to adjust.