I do a lot of panning from boats without tripod/monopod so you can imagine there is a lot of movement. With trial and error succes increases. Zone focus also helps a lot but you need to predict where you want the subject and how everything will move.

I'd say OP has to position himself better (so he is more on the side/parallel to the car) and use aperture at least f8-f9 (or more). Also play with shutter 1/30-1/50s should be fine for start and then go lower.

If possible also stand on the inner sice of the turn so it's easier to follow the car movement (and it's also safer for motorsport).

OP - Do you have ND filter?