It's about someone who's very emotionally fragile - she/he has a "paper heart" and isn't true to her/himself to keep from disagreeing with someone, maybe a love interest?
It reminds me of someone who defines themselves by the relationship they're in, and when that relationship ends, they lose their identity ("And who are you now, when you don't know where to start?") The whole "push the brakes down" thing is referring to this person going from relationship to relationship very quickly to avoid being alone, and they're telling her/him to slow down, give your heart a chance to heal and figure out who you are before you end up back where you started again.
It's about someone who's very emotionally fragile - she/he has a "paper heart" and isn't true to her/himself to keep from disagreeing with someone, maybe a love interest? It reminds me of someone who defines themselves by the relationship they're in, and when that relationship ends, they lose their identity ("And who are you now, when you don't know where to start?") The whole "push the brakes down" thing is referring to this person going from relationship to relationship very quickly to avoid being alone, and they're telling her/him to slow down, give your heart a chance to heal and figure out who you are before you end up back where you started again.