I've got a mesh that I plan on rigging for animation. It's a complete figure, and it has perfect symmetry. While creating vertex groups, in Edit mode, I am going through the painstaking process of selecting every face that should belong to that group, for instance, upper_leg_R. When I want to select all the faces to establish upper_leg_L, I should be able to just select the opposite of upper_leg_R, but I can't figure out how to do that. When I select "Select Mirror", I get a yellow error message at the bottom that reads "0 faces mirrored, 63 failed".
I don't want to modify the mesh at all; i.e., I don't want to add or delete any part of it. I just want to select the symmetrical opposite of my current selection. How do I do this?