I'm going to leave my girlfriend for somebody she hates
That better be a joke, why would you leave her for someone like that?
I'm not sure, I think there's something wrong with my head. It feels like it's going to explode.
You're free to end a relationship and date whoever you want, but leaving your girlfriend for someone she hates is particularly cruel. There are plenty of fish in the sea, you don't have to immediately go out someone that your ex hates. That's immature and will make you look like a bad person.
No thanks. I'm going to do it and I don't give a crap about what you say.
I'm just saying, don't you care about her feelings one bit? You were in a relationship, so the loving person is within you somewhere. Why not just break up peacefully and wait a bit before going out with someone else, especially someone off limits?