But if treating your room is out of the question, I recommend you just get accustomed to how things sound in your room, constantly reference albums you like. Also, take your mixes to your car and listen to them, get used to how your mixes relate to the real world, and adjust accordingly.
standard treatment would most likely look like: diy bass traps (rigid fiberglas or rockwool) in 4 corners from low to ceiling, diy broadband absorbers (rigid fiberglas or rockwool, there COULD be a foam for broadband absorbtion....) to left, right and ceiling. your room doesnt look like its huge so a diffusor COULD be of high priority after the two things metioned above. further info ->link