I think weakness is right. It's definitely about a relationship. It's about knowing something that your partner has done secretly and going through the denial of it all. She can see how lifeless the relationship has become and it's because of something her lover has done. Possibly she literally was "quietly half way in the room" when she caught him and he couldn't "see behind him" to find her watching. When she says she can see behind herself, she means that she can see through her feeble attempts of denying what happened or her postulated and persistent belief that he still loves her and realizes that it is all just an attempt to preserve her relationship and that it's easier to keep it inside than speak it and face the pain of the truth. Now all she can do is question herself about what to do (stay or go) and daydream about all the things she could do (confront him, stay, go, remain silent, seek revenge, etc).
I think weakness is right. It's definitely about a relationship. It's about knowing something that your partner has done secretly and going through the denial of it all. She can see how lifeless the relationship has become and it's because of something her lover has done. Possibly she literally was "quietly half way in the room" when she caught him and he couldn't "see behind him" to find her watching. When she says she can see behind herself, she means that she can see through her feeble attempts of denying what happened or her postulated and persistent belief that he still loves her and realizes that it is all just an attempt to preserve her relationship and that it's easier to keep it inside than speak it and face the pain of the truth. Now all she can do is question herself about what to do (stay or go) and daydream about all the things she could do (confront him, stay, go, remain silent, seek revenge, etc).