There are a number of devices that can make getting in and out of the car easier. You can find them at medical supply or auto stores or online.
And if you don't already have one, think about getting a disabled parking permit. Your doctor can help you. He or she will fill out a form that you can take to the local licensing or tax office. These permits may be permanent or temporary.
Grab bar and door strap
There are several types of handholds that you can install on the frame of your car door or beside the door.
They can give you something to hold on to as you get in and out of the car seat.
A swivel seat is like a lazy Susan or a turntable. You sit down facing sideways and then use it to turn forward as you pull your legs in.
Seat belt extender
You may have a hard time dealing with a normal seat belt. An extension may help you find and reach the end of the belt more easily.