I'm not going to cut my hair the way my girlfriend wants
Why aren't you going to listen to her?
I want to be different in this way.
If you don't want to you don't have to. It's up to you how your hair looks.
I don't care, I want to look different than her. At least while I'm younger.
That's great you are standing up to yourself. Your girlfriend is not you, so she can cut her own hair if she wants to. Your happiness matters a lot, and if you don't like her suggestions you don't have to take it.