Maybe it's the slanted ceiling?
It might give a bit of a cramped feel.
IDK what time of the day you took the photo, but does the sun actually ever directly shine into the room?

I dunno if it works, but maybe if you're in a darker area of the room, the light from the window might make the overall room seem brighter.  If that does work, perhaps move most of your stuff to the sides so you have an empty area in the middle.

Personally though, I like darker places >_<