I thought the ending made sense in light of **SPOILER**
I thought the ending made sense considering how inexperienced she clearly was as a therapist, even as a medical student and also with regard to her "mentor".
It reminded me of movies like The Village and The Machinist, each of which had endings that were explainable... totally different from The Ring in that sense...
I would tend to agree that Don was in her head - I didn't catch that the tv characters were the actors in the actual plot... In the end they show Don disappear near the end of the hallway so that would indicate to me he's fictitious.
Anyway the process that it puts the audience member through is engaging and interesting.
Some of the script I thought was not too developed but it didn't ultimately matter to me, I think it's worth a watch... After all the director was 23... I actually liked that there weren't a lot of bells and whistles, it just followed an idea through to its conclusion.