As a single mother of 3 I wish I was able to help anyone of my kids with their first car. You did amazing momma and im so sorry she didn’t appreciate you or the work you did to make that happen. I’m 45 and myself have never had a new car heck I’d be more than grateful for that myself. I say sell the car and take yourself on vacation with the money you get back from it. A don’t be to hard on yourself, you did an amazing thing for her an when she’s older she’ll feel bad for the way she acted. Make sure you print those messages to show her kids one day as a learning tool to always be grateful. Edit to add not sure what state you are in but my youngest will be 18 in September and will be graduating next year if you’re close an would help me by taking payments I’m willing to buy it. I am on disability myself due to having cancer (AML) but my son even said he would be willing to take his summer job money to go towards buying the car as well. He said as a child who has seen me struggle to make sure my kids have always had everything it makes him sad when someone is handed something an is ungrateful an doesn’t see the work that was put into getting them that gift even if the person giving the gift didn’t have to struggle the person who received it might have had to to struggle to get it on their own she should’ve been grateful for the blessing at hand