A group of teenagers has created an app that offers an antidote to Britain’s “share everything” social media culture.

Memory Star allows users to create happy memories in a “virtual memory jar”, which they can revisit at any time to give themselves a boost when they are feeling down.

These memories – which can be uploaded photos or messages and appear as constellations on the screen – are not for sharing with social media “friends” on Facebook or Instagram, but are purely personal reminders of happy times.

The app was created a group of five 15-year-olds from St Paul’s Catholic College in Burgess Hill, Sussex – Sacha Botting, Dominique Froud, Jack Gumm, Gemma Kelly and Zuzia O’Donoghue – who drew on their own experiences of the pressures of school, friendships, home life and social media to come up with the idea.

O’Donoghue said: “There’s so much pressure to present a very happy image on social media. There’s something very competitive about it. Who can get the most likes? Who has the nicest life? You have this sort of perfect persona that you’re putting forward of yourself, but you don’t always feel like that.

“With Memory Star, you just look over all your memories without worrying or being self-conscious about whether people are going to think you look great or not. This is just to make you happy. You keep it for yourself and no one else ever has to see it.”

Botting, who kept a glass jar full of scraps of paper in her kitchen, which she used to cheer herself up when things were getting too much, inspired the app. “When you are not feeling great, it’s easy to forget the compliments you have received and happy memories,” she said. “Memory Star reminds you of them and gives you a confidence boost. It has definitely helped me.”

After beating 900 other students aged 14 to 16 to win the Astellas Innovation Challenge last year, the app was professionally developed and launched last week.

The competition was set up by the pharmaceutical company to encourage young people to engage with science, technology, engineering and maths by designing an app that supports healthy living.

The impact that social media is having on children’s mental health is a cause of increasing concern. Last week the Office for National Statistics warned that children who spend three hours or more on social media on a school night are twice as likely to suffer poor mental health.

This finding is supported by a new YouGov survey for Astellas of 885 people aged 11-18, which revealed that four out of five (81%) have recently experienced negative pressures, ranging from pressure to perform well at school (53%), to uncertainties about future education and career (50%), worries about body image (44%), family conflicts (32%), pressure to look and behave like people their age (28%) and bullying – both at school (13%) and on social media (5%).

Two-thirds (65%) believe there is more pressure on them than there was on their parents at the same age – rising to 84% among 17-year-olds. To deal with these pressures, half (49%) go online, while 40% talk to their friends by text, phone or social media, and 38% play computer games. Only a quarter (27%) talk to their families, while 19% play sport to alleviate stress.

However, a third (32%) worry about receiving negative responses to posts about happy thoughts, feelings or experiences on social media and, of these, 71% said this would make them less likely to post about these in the future.

Nihara Krause, consultant clinical psychologist and founder of the teenage mental health charity stem4, said: “Young people today have to cope with a whole range of pressures that their parents didn’t. As well as concerns about school and their future career prospects, there are new sources of stress in a digitally connected world. While the internet can connect young people with their friends, it can also put pressure on them to present a perfect image of themselves.

“We all have times when we’re not feeling so great and happy memories can be a source of huge comfort. Research shows that the boost you get from looking at a photograph that reminds you of something positive is more beneficial than a bar of chocolate or a glass of wine.

“It’s very refreshing to see an app that emphasises the personal nature of memories, without the pressure to share everything with the whole world.”

Although Memory Star is aimed at teenagers, its simplicity makes it appealing to all ages. “It really could be applicable to absolutely everyone,” says Kelly. “We forget we’re not the only people who have stress; my 10-year-old sister loves the idea and I’m sure adults will find a use for it too.”

Memory Star is available to download for free from Google Play and the App Store