The cafe woman -- what was at the root of her alcoholism and anxiety?

Anyone have any thoughts on this? When at her work, she seemed quite relaxed and content (albeit, there was an underlying hint of a lack of self-confidence and happiness in general; it was ever so slight). However, when she arrives home from work, she immediately opens a bottle of Vodka. Was she just desperately unhappy and lonely? I surmise she may have been lonely, as she invites Johnny -- a complete stranger to her and someone whom she has certainly never met before -- but there was just something else about her that was impenetrable and difficult to deduce (admittedly, Leigh doesn't give the character enough screen time for any development to take place and, consequently, the viewer a chance to deduce what or where her problems stemmed from). She lived in a house that wasn't hers -- the house was owned by a gay couple who were on vacation -- and once Johnny begins to get to know her a little better and talks to her more, she suddenly seems very anxious and frightened (there's a scene in one of the rooms of the house that was replete with ornaments and portraits, where, suddenly, she looks terrified for no reason while Johnny is talking to her) . Why was this? Possibly due to a previous abusive relationship?

Your input is appreciated.