Children are making more calls for help after being "cut off" from support networks during lockdown, the NSPCC said.
Almost 43,000 counselling session have been delivered by Childline since restrictions were introduced in March.
Most related to loneliness and low self-esteem, the NSPCC said.
A Birmingham teenager has written a book about being bullied and is now campaigning for schools to prioritise pupils' mental health.
"The pandemic has cut children off from the reassurance many of them need," Childline founder Dame Esther Rantzen said.
"When young people are facing mental health issues such as anxiety or depression, or are struggling with eating disorders or self-harm, they often hide it from their parents and families."
The NSPCC, which runs Childline, said a spike in counselling sessions for children with eating disorders and body image issues was found, rising from an average of 335 calls a month before lockdown to 443.
Tegan, a 13-year-old girl from Birmingham, was bullied at school for four years and is now working with the children's charity.
"If anyone is being bullied and they can't talk to their mum or dad, then they can call Childline," she said. "You can talk to them about anything.
"I want to share my experiences so other children know that they aren't alone."
Dame Esther said children were likely to feel "especially isolated" during lockdown.
"If these tough times have caused children to feel an extra level of anxiety, we want them to feel confident to express their fears and share their worries," she said.