"You don't necessarily know it is happening, but it's like death by a thousand cuts."

From the outside everything seemed perfect for Sarah Williams. A graduate job in banking, seemingly settled in the glamorous London lifestyle and living the life that many people aspire to. But Sarah knew something was not right.

She said: "It just got to this point where it was a grey day. It was miserable and it was cold, I just hadn’t been outside. Not only was the weather grey, but I was grey. I was just losing my soul."

From that point on, Sarah changed her life completely and is now best known as the founder of Tough Girl Challenges in which she tackles a number of extraordinary physical challenges around the world to inspire young women to live life to the fullest. 

For Sarah, it has been a journey centred on mental resilience and strength.

When she was working in central London, even though she felt like she should be happy with her life, she could not bring herself to smile.

"You can’t really enjoy your Sunday because you’re so worried about the week ahead, or you’re celebrating getting to Wednesday or Thursday," she said.

"I’m a very positive person and mentally it was like ‘I should be enjoying this’. But I knew I needed to remove myself from that situation.

"It was when I was over in South America backpacking that I finally got this opportunity to really think about what I wanted to do with my life. I asked myself questions like 'Is this the direction I want to take my life in?' Having that free space and that time to think was very powerful. I realised I like challenges, travel and adventure."

And boy, does she like these three things! 

In the intervening years since that poignant moment of self-realisation, Sarah has run the London Marathon five times, completed the Appalachian trail, which is equivalent to going up Mount Everest 16 and a half times and finished the Marathon des Sables in the Sahara - rated at one point by the Discovery Channel as the most difficult footrace in the world.

What do these challenges mean to Sarah and why does she put herself through excruciating pain, both mentally and physically?  

"I think it is about challenging yourself and stepping outside your comfort zone. There’s also a selfish reason for doing it because you want to see how far you can grow and develop, to push yourself and see how much you actually wanted to do.

"It gives you these new experiences which you can then use in other parts of your life."

It is one thing to say you will undertake a challenge like a marathon, it is another to actually do it. The hours of physical training needed is extraordinary but the mental preparation is equally important. Everyone has their own approach, but Sarah's is particularly interesting.

"A practical thing that I do is something called a ‘what if?’ One of the best ways to describe it is you have a piece of paper, you draw a line down the centre and then on the left hand side of the paper you write every single fear that you have," she said.

"Before the Appalachian trail, the fears were how do I deal with bears, what if I get blisters, what if I get injured, what if I get lost, what do I do if I run out of food and water?

"I’m not saying you can count every single thing, but hopefully you can go through 80 per cent of the things that could happen so that when you are actually out there in that situation, it’s not a complete shock or new because mentally you have already been through it.

"Another really practical thing that I did for the Appalachian trail was knowing my reason why. For me, I wrote down every single reason why, whether it was personal, proving people wrong or wanting to inspire people. I ended up having this big list so sometimes when you have those low moments, you can look back at why you wanted to do this challenge."

Mentality is also key when actually undertaking an event or challenge and while going to the peak of Mount Kilimanjaro, Sarah came to the realisation that negativity was not going to help.

She said: "I realised that I am the only person in control of my thoughts. I’ve got to be in charge of what voice I’m listening to and which one do I want to feed. I had to be my biggest cheerleader and I needed to be supporting myself.

"When you can flick that switch [from being negative to positive], it’s an incredibly powerful thing."

The importance of mental approach is something Sarah has noticed on her Tough Girl podcast, where she invites guests who complete astonishing challenges to talk about their journey.

She said: "It’s really interesting how exercise and being part of nature can help with people’s mental health.  

"The thing is when women and girls go out and do physical challenges that they don’t think they can do, when they accomplish that goal, they build internal self-confidence."

Sarah realises that despite the great strides that have been made, there will still be people out there who face a grey day, not too dissimilar to what Sarah faced all those years ago. She has a message for them.

"There is no shame in asking for help, or in admitting that you are not happy. We just beat ourselves up all the time in trying to have this level of perfection but sometimes all we have to do is to be kind to ourselves. Treat yourself how you would treat your best friend."

Sarah was speaking to NewsChain ahead of World Mental Health Day on 10 October 2019. This year's theme is suicide prevention. More information on this year's theme can be found on the World Health Organization's website.