It may not come easy to you, but be as indifferent and short as you can possibly be, make sure she is the only one interesting in "talking" (obviously thats not her interest, just trying to regain lost controll). And seriously, 21 hours flight is as awkward as only you make it, semi-ignoring someone in public transport isnt that hard (its natural trait here in sweden lol), sleep, just put your head phones on or read a book.