Blender pose mirroring help

Helo I’m trying to port a model from Bloodborne to Skyrim with Blender, and I’m posing the rig to fit the Skyrim reference shape. I can pose one side fine but when I;

  1. select all of the left side bones that I’ve posed
  2. copy the pose
  3. invert selection so that only the right side bones are selected
  4. paste x-flipped pose

all I get is the same pose applied to the bones I copied out of. It doesn’t alter the right side at all. Even doing this a few different ways to try and mix up the steps, not inverting the selection, only copying the parent bone like the thigh or clavicle always gives me the same result of reapplying the pose to the already posed side.

Googling the issue wasn’t any help since those people got the pose to apply to the opposite side wonkily, which is what I can’t get done. If needed I can drop screenshots and the project file, I’m on 2.79

ps. I didn’t see a dumb questions thread otherwise I would’ve posted this there.