carsick/anxiety?

my cat takes 12 car rides per year, max. she comes with me to college and then back home to visit family on breaks. 1/3 of the time she is okay during our car rides, meows a bit in the beginning and then sleeps. but the other 2/3 she screams the whole 4 hour car ride and/or throws up. what can i give her to help calm her down? i could take her to the vet but last time i brought it up the vet said she was still young and would get used to car rides.

is there a medicine i could give her that doesn't need a prescription? maybe melatonin? i just want her to be more comfortable in the car and not scream. if my best option is to go back to the vet and ask for medicines for anxiety or nausea, i will. but i wanted to see if i have any other options.

for the bot: 21 years, USA, vet not easily accessible, not recently contacted, main issue carsickness/anxiety, cat 3 years old, spayed female, low income but have money saved for vet emergencies.