You don’t need to worry about anyone else’s opinion – if the car isn’t for you, you are absolutely 100% free to cancel the order at any time until you enter your PIN number which you should only do at the point of being completely sure you are happy with the car and are happy to drive it away.
Tell both the dealer and Motability as soon as you can, and if they ask simply say “The reasons are personal and I don’t wish to divulge them, I want to exercise my right to cancel in accordance with the official Motability rules” and don’t let them talk you into anything you aren’t happy to do.
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."