It is clear that the theme of this poem concerning growing up deals with both the difficulties of growing up but also the need to take responsibility for one's own life and to not be bogged down by the problems that beset us. Note how this theme is presented in the first stanza:
Bottled up inside
Are the things I never said,
The feelings that I hide,
The lines you never read.
It is clear that the speaker is somebody who is experiencing significant internal problems, feeling that they have to "hide" their emotions and not say important words they feel should be uttered. This results in their feeling of being "bottled up." However, as the last stanza details, it is clear that the speaker is not going to remain like this, and is determined to make the most of their life and not remain in this state:
I'm off to find, my world, my dreams,
Carve my niche, sew my seams
Remember as I sail my seas,
I'll love you all the way.
There is a determination, shown by the repetition of "my" in this stanza, for the speaker of this poem to live their own life and to make it work for them, in spite of the difficulties that they will face and already have experienced. This indicates the theme as being one that concerns not just the problems that are faced by adolescents, but also the more hopeful theme of growing up and taking responsibility for one's own life and actions.