My theory about the monster

Apologies if this has been stated but i felt like the monster was her alcoholism. The flashbacks seemed to focus pretty clearly on the effects her drinking had on her ability to be a parent. The monster pursued her relentlessly and destroyed those who attempted to help her. It thwarted any last-ditch efforts she made to redeem herself and connect with her daughter. The child was ultimately faced with confronting something she did not understand that would claim the life of her mother.
I've even heard people in substance abuse communities refer to their addiction as 'the monster waiting outside your door'. So i don't know, that's just what i got from it. Gracias.