In a witness statement read out ahead of the sentencing of Becky's killers at Bristol Crown Court, Tanya Watts said she was left with only "continuing nightmares... and her grave to visit".
Nathan Matthews, 28, was sentenced to 33 years for murdering his stepsister in a sexually motivated kidnap plot.
His partner, Shauna Hoare, 21, was jailed for 17 years for manslaughter.
Ms Watts said the details of her daughter's death were "unbearable".
"Becky has been robbed of her future and we have been robbed of all those future milestones," she added.
"Every day has been a living nightmare and if that wasn't bad enough, for some reason that I will never ever be able to understand, after my daughter was murdered, she was mutilated, cut into pieces.
"It's like the worst of all horror movies but this is real, this is my child. She was only 16."
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In another witness impact statement, Becky's father Darren Galsworthy described what happened to her as a plot that was "borne out of hatred, jealousy and greed".
"They [Matthews and Hoare] sat in our home knowing what they had done and watched my very public descent into madness and despair.
"They said nothing and carried on with the pretence of helping us and showed no emotion at all."
"Not only did they cruelly rob Becky of life, they also took her dignity in death. When I close my eyes to sleep I see Becky's death over and over again. I see what they did to my child."
Matthews suffocated his step-sister at her home in Crown Hill, Bristol, on 19 February.
He moved her body to the home he shared with Hoare, less than two miles away in Cotton Mill Lane, and in the days that followed he used tools to dismember and package Becky's body.
Hoare was also convicted of perverting the course of justice and preventing a lawful burial, charges which Matthews admitted.
Becky Watts' mother has said that seeing her murdered child in the mortuary still "haunts" her.