I'm going to tell my girlfriend she needs to make more money
Why would you tell her she needs to make more money?
Because she can do better for me
You should not pressure her into making more money unless that is what she wants. You should not tell her how to feel about the money that she is making now.
I just want to motivate her to work harder.
Well it's up to her if she wants to make more money, not you. You shouldn't pressure her into it.