@cookiemonster it's not really that simple. It might look very obvious. Break up with your girl and be with the one you love. But what do we know about Kido so far? It's that he doesn't really wanna cheat because he has a girlfriend. It was shown many times when he still had no feelings for Emiru.
Being in a relationship means you hold the trust of your partner. You're responsible. You don't want them to be hurt. Even if you're in love with a new person, you still don't wanna hurt them.