I'm really surprised you can get child support until age 21, here it's 18 and I think that is as it should be. An 18-year-old is legally an adult, no longer a child. I can't imagine an 18-year old not having a driver's license. Any kid I've ever known is taking driver's training at age 15 and can hardly wait until the day they turn 16 and go get their license. Our kids both started jobs at age 14, we drove them or they rode with friends' parents. By age 16 they were both working part-time jobs the year 'round, and we bought older cars for them to use while they were still living at home. DS got married at 18, so our monetary obligations to him pretty much ended at that time (although we have helped out occasionally here and there). DD went to college and did use the car we had for her during that time as well, but she paid for her own gas (we paid insurance).
Our DS is divorced and pays over $800/month in child support for his 11-year-old son. He does help pay for extras, but at age 18 I'm sure most of that will stop except for helping with college expenses. I don't think parents are obligated to pay for a child's college costs, if they CAN then that is great, but if they can't that's okay too. There are grants/loans for that purpose.
I think you're very lucky to be able to get "child" support until your DD is 21, if it were me I don't think I'd be asking for many extras, especially when the "child" isn't contributing anything herself. If anyone should be helping you to pay for gas/wear and tear on your car driving your 18-year-old around it should be her.