While I can't claim to be an expert in acoustics, I do have a bit of educational background in the field. I can say with 99.9% certainty that you have major room modes issues combined with some sort of comb filtering caused by reflections. The room modes would be the cause of the discrepancies you're seeing in the low/low-mid frequency range (pretty much anything below 200Hz). The reflections would be the problem for anything above that. It looks like your low end is absolutely fucked so you should definitely deal with that first.
 
Room modes occur when the distance between surfaces is (or an integer multiple of) the wavelength of a certain frequency. This causes the waves to bounce back any forth between those surfaces causing 2 problems: 1. There will be a boost in intensity of that frequency at points in the room where the wave is at an antinode (peak) and drastic cuts where it's at a node (point of zero-crossing) 2. The wave will self amplify and therefore it will take longer to decay. While room modes are unavoidable in any room, some rooms are more problematic than others. There are many devices that can be built to deal with room modes such as bass traps but any of them must be designed specifically to deal with the problem frequencies.

I'm currently in the process of studying the field more in depth and would be glad to help you out with your room if you'd like. Just to confirm my theory though, could you please play sine waves of 30, 85, 110 and 140Hz and walk around your room to hear if you notice a difference in volume between different areas?

Ok if we're gonna do this we might as well do it right. Install this software: http://www.roomeqwizard.com, run a measurement at your mix position and upload the resulting files. Make sure you record the noise profile of your interface and calibrate levels before doing so (it's all explained in the tutorial when you first open it).

Also if that's at all possible, reconfigure your room so that your speakers are facing the length of the room. That will lead to longer bounce paths and therefore less reflections than with your present configuration.

that mirror is like the perfect hi freq reflector, where on the other side you have shades with many seperate blades that are at an angle and help diffuse the sound.  This could cause high end comb filtering.