It's a song targeted at tweens, so the lyrics are generally straightforward. The day before ("can we bring yesterday back around"), he probably told her he loved her, and she couldn't say it back, or wasn't ready to do so, which "let [him] down", and made her feel "dumb". As a result, she hasn't been able to stop thinking about it ("not a day passed me by when I didn't think about you").
However, now it's the next day, and "[she knows] how [she] feels about [him] now", and is therefore, ready to tell him.
It's a song targeted at tweens, so the lyrics are generally straightforward. The day before ("can we bring yesterday back around"), he probably told her he loved her, and she couldn't say it back, or wasn't ready to do so, which "let [him] down", and made her feel "dumb". As a result, she hasn't been able to stop thinking about it ("not a day passed me by when I didn't think about you").
However, now it's the next day, and "[she knows] how [she] feels about [him] now", and is therefore, ready to tell him.