Bellon is only 16 weeks but has just started to jump into the back of my van by himself
Dad told me a story of a German dog trainer who came to live in England and for the sheer fact of being German, obviously, everyone thought she was the most magnificent dog trainer ever.
However, she fell into the same trap as you. Had a dog from a puppy, lifted it into the car every day until it was about 12 months and then it refused to learn how to jump in.
This dog was almost bigger than her by that point...
Just goes to show even hardened supposed dog trainers have the same problems as everyone else.