It marks the moment you are about to do something and the feeling that comes with it, which the rest of the sentence supplies: leaving the school you have grown attached to and feeling sad, having to cut it down to three songs and finding it hard, going in for real and getting nervous. The word 막상, meaning when it comes down to it, sits in front of it in two of these lines. Two other lines use the set phrase 그러려니, which means to shrug and accept something as simply how things are.
Register: neutral; the 그러려니 phrase is very common in speech