But I do believe in the girl/boy brain in the wrong body bit because of my own experiences.
Your brain is part of your body. It's not possible to have a brain in the wrong sex body from a scientific point of view. You are describing your own subjective experience which might help you to make sense of your own feelings, but there is no objective scientific evidence for this.
It is possible to not identify with the stereotypes imposed on you because of your sex.
It is possible to feel revulsion about part of your body, or a feeling that it doesn't belong to you. This is a psychological or neurological disorder.
But wrong-sexed brain for your body? Not literally possible.