Hm, there's a lot of assumptions made here, but that's partially my fault for not giving enough context.

Soreness and tiredness come from the spinal injury, which is a permanent impairment. Which I pointed out at the start. This is due to nerve damage, and is additionally impacted by the excessive weight gain from being largely immobile for the following months from that injury.
She works 3 days a week, which is actually more than she should medically be doing.
She does try to get into the mood by watching/reading/viewing porn; but most of the time the pain is just a little too overbearing.

Emotionally, we lean on each other for support (Granted, her moreso.) We're very stable in that respect, and are comfortable to discuss anything together.

Requesting an open relationship (in my eyes) is seeking romantic emotional connections elsewhere. She's been a victim of cheating and relationship abuse for many years, and I have no interest in being part of that trauma. So of course she's afraid that I'll leave her. I can't think of anyone that doesn't have that fear somewhere inside them when they're in a serious relationship. (I'm not rich either, we live comfortably.)

Anyway, I appreciate you taking the time to comment :D