It seems, to me, that the narrator believes that this girl he used to be with belongs with him. He doesn't think that any guy will really love her as much as he did, and that if she ever feels like something is missing, it's because she must have once felt complete with someone else (the narrator). She must not seen it that way, and so the narrator wants her to let him know if she ever finds someone she trusts, cause he doesn't think she will.
It seems, to me, that the narrator believes that this girl he used to be with belongs with him. He doesn't think that any guy will really love her as much as he did, and that if she ever feels like something is missing, it's because she must have once felt complete with someone else (the narrator). She must not seen it that way, and so the narrator wants her to let him know if she ever finds someone she trusts, cause he doesn't think she will.