To say that balancing Tabitha's split personality, which could rapidly shift with less than a moment's notice, was difficult could perhaps be the understatement of a century. To simply put, Zane had to manage his companions carefully. Withdrawing them at a time when Tabitha was sane.
Because if she was in her possessive and crazed state when Gwen, Eliza, and the others were around it would have resulted in the level of destructive jealousy that only an enraged deity could inflict upon the earth.
So, because of this, Zane was constantly on his vigil around Tabitha. The more sane side of her began to realize throughout their journey together that what Zane had told her was the truth The gaps in her memories between her shifts in personality, as well as the new location which she found herself every time she entered a state of sanity was more than enough evidence to support the theory that she had two split personalities that did not share memories.