I agree you should be able to love who you want. What you may not know is that the Jehovah's Witnesses are not a religion like any other religion. They are a cult, and controlling who you choose to marry is just one way they have of controlling you. I was a JW for 28 years, until I realized they were not what they pretended to be, so I left.
The problem is that your girl probably believes this is the one true religion and that if she doesn't do what they say she will die at Armageddon, which is happening any time now (and has been for over a hundred years). It's very hard to fight that kind of brain washing.
You are probably better off finding someone who isn't in a cult.