I'm going to make my girlfriend pay half the rent even though I make more than her
I'm not sure if that is fair, do you believe that it is?
No, I want the other half.
It's fair for her to pitch in. But if you're doing fine and she's struggling you should be more considerate and fair.
I've done things for her, I bought her a necklace for her birthday.
It's wrong to try and justify extorting money. I hope you are able to take her perspective and understand how it is hurtful to her to have this happen.