live in a world of (one's) own

To be consumed by one's thoughts, imagination, etc., to such a degree that one is unaware of or insensitive to one's surroundings and/or the outside world in general. Helen lives in a world of her own, so it's very difficult to talk to her about practical issues.

be, live, etc. in a world of your ˈown

seem not to be aware of things happening around you; be a person who has ideas that other people think are strange: I’m not surprised he didn’t know about it — he lives in a world of his own, that boy.