Lisa Squire said that "I kept you safe for as long as I could", but that she was "so so sorry" to have lost "one of the most precious things in my life".
She wrote: "My baby girl is gone. Gone forever. No more birthdays with us.No more Christmas Days with us. No more family time all together."
Libby vanished in the early hours of February 1 near her home in Hull.
Despite a huge search and widespread publicity, no clues to her disappearance emerged over the following weeks.
Mrs Squire wrote on Facebook on Saturday morning that "no family should have to endure" what they had been through.
Footage released by Libby Squire's family during the search to find her shows the 21-year-old enjoying herself
"No sisters should have that special girl they have known for all their time on this earth taken from her," she said.
"No 11 year old brother should have his idol taken from him.
"No dad should have his first born little girl, his princess, taken from him.
"No grandparent should lose their first born grandchild.
"No aunt and uncle lose their first born niece.
"No mother should have to watch her family go through all of that."
Mrs Squire went on to say the worst point was having to tell Connor, Libby's boyfriend and the "absolute love of her life", the news.
She said her death had left so many people "broken", before adding: "I cannot thank you enough my darling Pie for making me a mummy.
"For choosing me to be your mummy. It’s an honour, a privilege and a joy."
Speaking on Friday, he revealed a post-mortem examination had been conducted but that they were withholding any results for operational reasons.
Pawel Relowicz, of Raglan Street, Hull, was arrested five days after Libby vanished and was questioned on suspicion of abduction.
He remains a person of interest, police have said.
He appeared before magistrates charged with a number of offences unrelated to Libby’s death and has since been remanded in custody.
Libby, who has been described as a popular student who studied philosophy at Hull University, went missing after going out with friends on January 31.
She was dropped off in a taxi near her home on Wellesley Avenue at 11.29pm, and was seen on a bench near Beverley Road around 10 minutes later.
She is believed to have remained in this area until around 12.09am the following day, February 1, but was not seen alive again.