On the cheap-to-free side of things, I would mix at very modest volumes or with headphones to minimize the rooms influence. Have you considered making d.I.y. bass traps using Owens corning 703 or rockwool, and maybe d.i.y.-ing some Diffusers out of wood or styrofoam? Both are relatively cheap if you do them yourself.
Check Ethan Winter or GIK acoustics, I think, or even Auralex, for suggestions as to how to mount them. With bass traps in the corners and Diffusers in a few key spots,maybe a couple auralex-type foam pieces at a few key reflection points, and you'll be in much better shape. The foam alone will help reflections, but you'll also have issues with comb filtering, excessive bass buildup and/or cancellation, and other issues that will plague a small room like this. I'm lucky in that my larger recording space lets me get further from walls and minimise some of this, but you got a lot of sound bounce going on in there to fight, and that makes it more work to treat.