hi arader and thanks for your post,
It may be that your dog finds riding in the car less enjoyable now that it is larger and cannot find a stable position to sit in. You could try filling up the foot space in the back seat with things such as a cushions and blankets so that the seat and floor space are even and your dog has a large flat surface to lie on.
It could also be that your dog had an unpleasant experience in the car at some point, maybe because it was ill or got a fright for some reason. You can start desensitizing her by getting her into the car with the doors all open and then rewarding her with treats for getting into the car. When she will regularly get into the car comfortably in this way start closing the doors when she is in there. Treat her and praise her when she is calm. Once she is regularly happy with this you can progress to turning on the car engine, later short then even later longer, drives. Keep sessions short and regular and you should soon see progress. I hope this helps and if you have any further questions please don't hesitate to ask.
All the best,
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