Dear Aswathi:
My advice: explain to her that her complaining is distressing you because there is nothing you can do about her unhappiness beyond what you already have done, and because what you already have done made no difference to her. It causes you to feel helpless, angry.. (you know what you feel, so express that). Explain to her that it is not responsible of her to complain a lot, because it is spreading her unhappiness and it is not helping her.
If she doesn’t respond to your assertion (above) with empathy for you, sincerely apologizing for distressing you, as she had, and if she doesn’t offer a promise to stop the complaining, then I would not, if I was in your place, have her visit me again.
anita