"
Been talking to myself forever, yeah
And how I wish I knew me better, yeah
Still sitting on a shelf and never
Never seen the sun shine brighter
And it feels like me
On a good day
This song might be about someone pushed away or bullied by society. She's been "
talking to herself forever", having no kindhearted company but herself for as long as she can remember. She wishes she knew better just who she was, rather than who people say she is. She feels "hemmed in" or forced to be someone others insist she is.
"
Sitting on a shelf" might refer to her abilities or the true personality she doesn't have a chance to show. It may also mean her in her isolation, as she's "never seen the sun shine brighter" and she's found some sort of solstice or happiness in her surroundings since she can find none in other people.
"
And it feels like me... On a good day" may refer to her having a good time some days despite being rejected by the rest of the people around her and not having anyone to share thoughts, feelings, and emotions with.