Demi Gardner, from Vale of Leven in West Dunbartonshire, said the situation in children and adolescent mental health services (CAMHS) was “completely unacceptable”.
Her son Charlie, 11, was referred to CAMHS in January 2020 and had a video consultation, but has had no contact since.
Scottish Labour leader Anas Sarwar said 8,873 children and young people have had their referrals to CAMHS rejected in the past year, while 1,248 have been waiting more than a year for their first appointment.
He said: “Even those who have had a first appointment are still not getting the treatment they need. Here is just one example. Charlie is a Primary 7 pupil. He was referred to CAMHS in January 2020. In April 2020, he had a video consultation with a doctor from CAMHS. That is the last time he heard from them. He’s had no diagnosis.
“Without treatment, Charlie has become withdrawn and doesn’t like to spend time with other children. His mother found a video he had posted to TikTok where he asked if anyone felt like they wanted to die because they were so different.
“Charlie’s mum told the CAMHS service this, but they said it would make no difference to his waiting time. They told her that it could be another two years before Charlie receives the support he needs. This not good enough. Charlie is not alone. There are thousands of children like him."
Ms Sturgeon said Charlie’s experience was “not acceptable”, adding: “It is the case that there are waits for CAMHS that are too long, but it is also the case that there is significant action being taken that is reducing, already, these long waits.”
The First Minister said nobody was denying “there is a significant issue here”, but added there had been reductions in the number of children waiting for treatment.
She said: “Does that say there is not still a challenge? No, it doesn’t. But it does say that the significant investment, the increase in the workforce, is now having an impact where we need to see it, and we need to continue that.”
In a statement, Ms Gardner said: "I feel that Charlie has been completely abandoned by the services which are meant to be there to support him. It is really difficult as a family to function knowing that he isn't getting the diagnosis and treatment which could make such a huge difference to his life.
"When I spoke to CAMHS recently, they said they were working on referrals from 2018, which is astonishing. That means, having had his difficulties identified early on in primary school, he could be in high school before anything is done.
"That is completely unacceptable. Our children are being failed and no one is doing anything about it."
Elsewhere, Scottish Conservative leader Douglas Ross demanded Ms Sturgeon improve “appalling” cancer waiting times in the NHS, saying one patient had waited two years to begin their treatment.
The First Minister said the median waiting time to begin cancer treatment was measured in days rather than months, a figure that had improved despite the pandemic.
She said: “There will be individual cases and sometimes clinical circumstances mean that it takes longer. And sometimes, yes, where failings in the NHS mean that it takes longer. The point I’m making is, for the vast majority of patients, that is not the case.”