Vergiss mein Ich (English title: Forget My Self)

http://www.pandorafilm.com/produktion/index_details.php?query_var=id&query_value=112

http://www.the-match-factory.com/films/items/lose-my-self.html

Jake mentioned memory as a way of determining self: This film is about a woman who forgets every single memory, and in the course of the film learns how to pretend to "be herself" again.

Interesting is also the name of the distribution company: "True Fiction Filmverleih".

Here the synopsis from the first link above:

Lena Ferben has lived almost half a lifetime when she loses all of her memories, the entire “I” of her self. Her husband is suddenly a stranger, her friends are unknown to her, her whole life is a fictional narrative. She must decide whether to become who she was or to remain someone else...

I recommend the description in the second link as well, it has much to do with the linguistic consequences of Lena's (main character) amnesia.

Unfortunately I cannot find a trailer of it in English – it would seem though, that it has been at the very least set up with subtitles, considering that it exists with an English title in the world of the internet.