Nas, you need to write down on how the car is affecting your health in almost every aspect, that your really unable to use to the extent you would like, so its a lump basically a lump of metal sitting in your drive, note you were only able to a minute test drive at the time. Once youve got every written down have a good chat with Motability and gell them what you have wrote down. They are a friendly bunch and are likely to allow you to swop as GG says.
Good luck.