She doesn't want to let go because she doesn't know what good could happen in the future and doesn't want to give up that possibility.
They say that time heals everything, and she wonders if it will eventually heal the relationship by waiting it out. However, if they wait for that to happen, what will they do to fill the hole while they wait? They're closed off from the Earth (other people) because they are choosing to stay together and hope it gets better with time, but she fears they'll waste their entire lives hoping it will get better.
To me, this song is about a broken relationship.
She doesn't want to let go because she doesn't know what good could happen in the future and doesn't want to give up that possibility.
They say that time heals everything, and she wonders if it will eventually heal the relationship by waiting it out. However, if they wait for that to happen, what will they do to fill the hole while they wait? They're closed off from the Earth (other people) because they are choosing to stay together and hope it gets better with time, but she fears they'll waste their entire lives hoping it will get better.