NTA.

You are legally and morally required to ensure anyone in your car wears their seatbelt for the protection of you and other road users.

Normally with a child, you put them in a car seat they can't get out of, and with an adult if they refuse you eject them from the car.

In your case, you can't do the latter because your daughter isn't an adult when it comes to being able to make safety decisions and you can't leave her.

That only leaves one option, a car seat. You have to treat her like a toddler to ensure she, you and others on the road are kept safe.

It sounds like you've done the right things in terms of testing all the alternatives, speaking to a therapist and ensuring she's kept entertained and isn't upset.

What else does your mother expect you do? What's her magical solution?