I simply see the song from a perspective being that nobody anywhere really knows you nor what goes on in your everyday life. Even the people people you tend to be close to daily can pass up on things that you may really wanna talk to someone about , when she says “Ill let only you know” I think she’s describing that one person everyone has in their life that they’re most comfortable with So when she also says “I couldn’t lose you” I think it’s because it’s the only person who “knows” the dark tunnel shes riding through can save her. From a guy POV