K, here's my interpretation. She's breaking up with a guy because he cheated on her or almost cheated on her. It's too late to beg for forgiveness. "You've forgotten how it started" refers to how they got together, possibly behind the back of another woman. Now she regrets it, sees it as a big mistake, once a cheater always a cheater. She's reminding him of what he's lost.
K, here's my interpretation. She's breaking up with a guy because he cheated on her or almost cheated on her. It's too late to beg for forgiveness. "You've forgotten how it started" refers to how they got together, possibly behind the back of another woman. Now she regrets it, sees it as a big mistake, once a cheater always a cheater. She's reminding him of what he's lost.