I'm pretty sure that MM has visualized many, many times the moment when William and Catherine will be sitting on actual thrones wearing actual crowns, being actual heads of state, to whom actual Parliament defer, with the PM being referred to as William's PM etc etc - all the deference she witnessed HMTLQ being afforded going to W&C. Attending the coronation would be a nightmareish dilemma for her: sit in the audience and witness all that, or be NFI and the whole world know it? (Only solution is to let it be known she was invited but turned it down, of course).
Meanwhile, she will be in Montecito trying to earn a living selling tat via her As Ever website, running her Instagram page with affiliate links, and trying to up listenership for a poor-quality podcast, with Harry moping around the house and a Prince and Princess unable to be free citizens.
An ego such as hers cannot, just cannot cope with such a situation. So close, yet so so so far. She knows the clock is running on becoming A Big Thing, she knows it's not happening, and she knows she'll be the lesser-than the second W&C have crowns put on their heads. It's where she's always been, what she's been trying to get away from all her life, and she knows she's heading right back there. The best time of her life will end up being around the time of the Oprah interview when Netflix and Spotify and PRH were banging on their door.
Hopefully she will focus on all the things she does have rather than the things she doesn't, but I doubt it. It's not going to end well, for either of them.