Matthew Daley, 35, denies murder but has admitted attacking 79-year-old retired solicitor Donald Lock claiming diminished responsibility.
Lynda Daley agreed at Lewes Crown Court the day Mr Lock was killed was "the day that all your nightmares came true".
Mr Lock was attacked on the A24 at Findon, near Worthing last July.
Earlier defence counsel David Howker QC said Sussex Partnership NHS Foundation Trust apologised to Mr Daley's family for having "failed" him in his care and treatment.
Mrs Daley told the court her son had many debts and the recent breakdown of his relationship with his girlfriend had left him "distraught".
"We were never provided with a proper diagnosis. We were frequently in meetings but we were not listened to," she said.
"It's been continual over the last 10 years. In the past 10 years we have pleaded to people, it has been on a regular basis."
Mrs Daley told the court the apology from the NHS trust was "10 years too late".
She said she feared her son was involved in the killing in Findon when she saw the news on television.
When she failed to contact him she drove to the stables where he worked and saw his damaged car.
The court heard the family decided to go to Worthing police station and as Mrs Daley was about enter, her son called her saying: "I need to know where you are and that I can trust you."
Mr Daley also told her he had done "something really bad or something really wrong.
"I killed someone."
The court also heard Mr Daley police told a police officer "when I drive, people try and break into my thoughts".
He also said: "It's sad that I had to kill someone who was so aggressive to me when I wasn't aggressive at all."
Mr Lock was returning from a cycling meet when he was stabbed 39 times during a "ferocious" attack on the A24 at Findon in July.
Witnesses have described Mr Daley as being "expressionless" during the attack, as though he was "having a passport photo" taken.
Mr Lock was fatally stabbed after he hit the back of Mr Daley's Ford Fusion and then asked why he had braked so abruptly.
Jurors have been told that Mr Daley's defence team will claim the defendant may have been acting in self defence because Mr Lock was angry and aggressive .
Consultant forensic pathologist Dr Nathaniel Carey said Mr Lock died from a stab wound to the aorta.
Stab wounds may have also also been inflicted when the 79-year-old lay dead or dying.
The case continues.
The mother of a man who stabbed a motorist 39 times told a court she "pleaded" with mental health experts to have her son sectioned.