My simplest interpretation is that Rachel mentions the mirror as providing a literal second person that she can meet, because that is being 'good to myself' compared to her previous experiences with real people; "...if I feel lonely I can alway talk with my reflection". This seems a tragic-comedy, sarcastic expression overall, perhaps ironically hiding an appeal to Martin to help her try a relationship again. I also feel that this mirror-idiom is so bleak that it emphasises the degree of loneliness that she really feels.