I'm going to leave my girlfriend for someone else
Is there something about your girlfriend that makes you want to leave her for someone else?
Let's just say I'm not happy with her and her behaviour. I don't like our relationship.
If you aren't happy, it's fine to break up. You shouldn't be in an unhappy relationship.
I'm going to Japan without letting her know.
It's alright you want to break up with her but it's not ok abandon her without letting her know. You should talk to her like an adult and make her clear that you don't want to be in a relationship anymore.