There's no telling a 12 year old that she needs to move on to a different horse. You can have her ride different horses, but she's always going to love Thor and he's always going to be her baby. The only thing you can do is flat out refuse to let her ride him anymore if there's going to be jumping involved. Tell her she's welcome to come out anytime and groom him and love on him, and that she can even ride him on the flat every once in a while, but that he's worked hard his entire life and he deserves his retirement.
I went through the same thing when I was her age with a pony named Iceman. I couldn't move up because he absolutely hated jumping, so I never got past 2'3" because that's as high as he would go. I ended up showing a huge appaloosa named Q for a while, but I would still ride Iceman whenever I got a chance.