To me, "the trick is to keep breathing" means that the girl is on the verge of imploding, of beginning to scream at the top of her voice, which she does not want : she wants to calm herself down, which she manages to do by trying to breathe normally : you know, it's like when you are really really upset, either on the verge of crying or of bursting with anger, your breathing becomes difficult because your throat becomes narrower little by little and you can't help it. And you tries to control your breathing, and while you manage to keep breathing normally you can control yourself (I'm not sure I'm expressing myself very clearly hum :/ ). To sum up, to "keep beathing" means to collect yourself and to calm down.
To me, "the trick is to keep breathing" means that the girl is on the verge of imploding, of beginning to scream at the top of her voice, which she does not want : she wants to calm herself down, which she manages to do by trying to breathe normally : you know, it's like when you are really really upset, either on the verge of crying or of bursting with anger, your breathing becomes difficult because your throat becomes narrower little by little and you can't help it. And you tries to control your breathing, and while you manage to keep breathing normally you can control yourself (I'm not sure I'm expressing myself very clearly hum :/ ). To sum up, to "keep beathing" means to collect yourself and to calm down.