The Lady MacGreggor (Patreon)

Celesse wasnt always in control of her body.
After the proto-shoggoth merged with her body, it began to change her, and in that time she was a passenger in her own head.
Eventually she learned more of Protea, and began to figure out how to exert her will over it. 

Eventaully, Celesse regained command of her body, though the shoggoth was now a permanent resident in her womb. 

It only took 175 years to regain control.