Nobody is perfect. How true. Despite the valid message, one wonders exactly what Jessie J (well, the songwriters) did to hurt this person so much. Could be cheating. "The trust is broken" line seems all too familiar to those who commit infidelity. And ultimately, what goes around comes around. The hurt that Jessie J inflicted on an apparent love interest inevitably came back around to her, and in the end, she also suffers.
However, it could be something less serious, as implied in the verses. She seems to have told a secret to someone she shouldn't have, and the person she's singing to is pissed. This is something anyone can relate to, be it a mature adult or a 10-year-old. We all have the potential to let our friends down. The chorus though is what I take issue with. Jessie J argues that because nobody is perfect, and because of this, the person should be able to forgive her. The forgiveness, like it or not, isn't up to her. What is forgivable to one person may be unforgivable to another. Regardless, the sentiment is real and the lyrics are self aware and reflective - kudos to the songwriters!