be straight-forward. Tell her you've been giving your ride for a year. you don't find that it's working out for you. You feel like it has become an obligation for your to give her ride. You need your privacy in your car and want to enjoy your car. So it's best that she finds her ride with someone else or take the bus.
Of course she's not going to be happy. It's normal and natural. If she says anything, don't try to explain anything to her, so argue with her for anything. Just say you feel like it works best for you.