Deny Existence was recently clarified to not work for this (much like Sister Mary's weakness no longer works with Deny Existence).
This has made this weakness the main difficulty in playing Diana, which is intended. She has a lower than normal horror value for a mystic, and depending on when this card comes out it can be absolute debilitating.
She doesn't have a good way to fish it out, and it is scenario failing to drop all of her willpower off especially if it comes out late. So the best strategy is to build her soak heavy and just hope you see this early. Brother Xavier is very attractive for this purpose, and also boosts her willpower.
After it's drawn you could play "Fool me once", but Diana will typically have low cycling power and it's unlikely you're going to run into this again. If you do find yourself somehow cycling rapidly with her you should consider Fool Me Once, as getting hit with this a second time will probably kill you.
Overall this is a swingy weakness, akin to Wendy's. Draw it early and it's probably just a foot note, Draw it late and you're going to have a bad time if you aren't prepared.