You and Apollo were dating. It wasn't easy being a God's partner, since his sense of right and wrong was a little distorted because of his father. Still, you loved him and would do anything for him. And hoped he would do the same for you too.
But one day, while coming back from hunting with Artemis, you found Apollo kissing another person. His hands were on her back, pulling her loser to him, as their kiss became heated, not even noticing you.
Later that day, he came back home, to find you waiting for him. You chose to not say anything at the moment, but to wait for him to come back home. And there ask about it, hoping for an explanation that would fix it.
Instead, he had a confused expression when you brought it up. Not understanding your anger. "If you saw her, why didn't you kill her?" Was one of his questions. "But, isn't it normal for couples to cheat on each other?"