What I think the stanza means is that she feels trapped, scared, and isolated, and uses the bird trapped in a nest as a metaphor for her situation. The bird symbolizes her, and the nest symbolizes what is holding her back. I felt the same when I was having a hard time going from school to school and kept feeling isolated.
I think that the second stanza is her seeing a way out, or something she wants, and that makes her want to leave and gives her courage. I also think this is a metaphor for her find something to fight for. A situation like that for me was when I started getting good grades, I felt like I was good and us full for something, and that has since made me try my best to get good grades.
The last stanza seems to represent her getting freedom, or her going against 1 or more things/people to get something she desires. I think that it shows her breaking free of whatever was holding back and getting freedom. I felt the same when I made friends.