She fell in love with the guy, but he is not aware of her feelings. Being around him makes her falling even more for him no matter how hard she tried not to.
She realized that it's hard for her to forget him since in her situation, it could be that he is a close friend.
In my opinion, 'your song' mentioned is what she imagined as his charm that draws her towards him.
That's why she sang 'you won't get to me if I don't sing'. It is simply her resolution towards her feelings for him, which means she's planning to ignore her affection.