In a smartphone message, her feelings are not conveyed. Even if I try to confess directly I can't even look straight into his face, so I put my thoughts on this piece of paper and I make up my mind before going to bed. She is troubled while listening to the voice of a sparrow.
She can't convey all her passionate feeling in a text message. She wants to tell him face to face, but she's too nervous to even look at him. She'll need to cram all those feelings into a letter. She was determined when she went to bed, but is back to fretting again with the coming of the morning.