There is a third party in our love, and my girlfriend is a little shaken. How can I stay?

As long as you are still yourself, it's enough to be a boyfriend. It's up to her to return or stay, and she can't stay after she leaves. If you try your best to keep her with him, and then let your guard down, she will hide her resentment against you and regret being with you. If she wants to cheat, it's only a matter of time.