Ordinarily, it is fine to cancel an order if something better / more suitable comes up, but this scenario is very different to the norm in that you have a 6 year old car that has had 4 extensions. That is beyond what is allowed.
If you don’t mind me saying, it sounds to me as though you really don’t want to change your car at all. If this is the case, have you considered buying your current car from Motability? The benefits that you have been sacrificing over the last 3 years have been significantly more than it would have cost you to run a car of that age privately. Motability usually ask silly prices that you can easily beat elsewhere, but given the age of your car they might be more generous.
If buying privately isn’t an option and you wish to stay with Motability, I’d make some time and get on and find something suitable. It really isn’t sensible to be paying as much as you are every year in benefits to drive a 6 year old car.