Crompsie, as you know, we've had similar experiences. (Though I would add that your case is far more hurtful than mine.) You know my thoughts are with you on this; keep strong.
I would think you have two options:
- Simply throw it out and not think about the matter again.
- Cut your last initial out of each sheet and send the stationery back to her.
Regardless, no matter what, don't think too much about it. I think you're right about number 5. And, yes, hug your cat.