Lorin LaFave said she was heartbroken and lost after her son Breck Bednar was stabbed to death at a flat in Grays, Essex in February.
Computer engineer Lewis Daynes, 19, admitted carrying out the murder after dramatically changing his plea on the day he was due to stand trial at Chelmsford Crown Court.
Barrister Richard Whittam QC told the court it was the prosecution’s case that the murder “involved a sexual and sadistic motivation”.
Speaking outside court, Ms LaFave said: “I am heartbroken and lost without my Breck and I will never be the same. He was murdered on my birthday this year and so much of me died as well. My poor triplets lost not only their adored big brother but also their mother.
“Breck was my amazing, clever, beautiful boy, a beautiful son who we will miss forever. He had so much potential that he will never now reach. He had so much to give to this world.
“I want Breck’s tragedy to open the eyes of everyone to recognise the dangers of online predators. It is a very real danger.”
Fighting back tears, Ms LaFave thanked Breck’s family and friends for their support and the police who “worked so hard on this tragic case’’.
The Breck Bednar Memorial Foundation has been set up to help raise money and awareness to help protect children, she added.
“I’ll never stop missing my Breck,” Ms LaFave said.
It is understood Daynes and his victim met after playing online video games together. Family members said Breck, from Caterham in Surrey, left home after arranging to meet a friend online.
He was found more than 30 miles (48km) away in Grays with a fatal stab wound to the neck on February 17. Ms LaFave and the victim’s father, oil futures trader Barry Bednar, who is from Houston, Texas, were both in court to hear Daynes’s guilty plea.
Flanked by two security officers, Daynes from Grays, looked straight ahead and showed no emotion during the hearing.
Daynes spoke softly as he replied guilty after the court clerk read the charge to him.
Mrs Justice Cox said that Daynes would be sentenced on January 12. Breck, who was a student at St Bede’s School in Redhill, Surrey, was an air cadet with 135 Squadron and attended St John the Evangelist church in Caterham, his family said following his death.
He was “extremely gifted in computing, electronics, and engineering”, they added.