Very few answers made any sort of sense, and all of them pointed towards flaws in her spell-craft. The first possible mistake was the Tantabus' limited ability to siphon negative emotions from her self-inflicted punishment. If she made the siphon too strict, it wouldn't be able to sustain itself over the long term. Was that why it tried to escape into the dream realm? It was starving and tried to find another source of anguish before expiring. 