... it doesn't exist.
One in four people will suffer a mental health problem/illness/issue at some point in their life/the past week/the past year (delete as appropriate) is a statistic seen all over any type of mental health media. It has never sat well with me. Mostly because I always think if an individual is unsure about their current state of mental health, it is easy to read that statistic and be reassured that 75 percent of people will never have a mental health problem... ever. And go ahead and place themselves in that 75 percent 'I'm going to be alright' bracket.
I'm not sure mental health problems work that way. For instance, if I were to say to you 1 in 4 people will suffer a physical health problem at some point in their life, you would look at me like a crazy person. It is quite clearly 100 percent unless anyone has figured out how to 'cure' mortality - or vampires exist, one of the two. We all get colds, and suffer from the various physical problems caused by ageing. There is a spectrum of severity here, but all are bannered underneath physical health nonetheless. Why are mental health problems treated any differently?
One dull Sunday mild curiosity got the better of me and I decided to look at where this stat came from. After a simple google search and on the first page I came across this article from 2010  in the Guardian. Jamie Horder writes that the there doesn't seem to be any hard evidence at all, and closest evidence he could find was from the 2007 Adult Psychiatric Morbidity survey or APMS  (often cited as the source of the 1 in 4 statistic) which quoted a value of 23 percent for participants who reported a common mental health condition over the last week. Although, astoundingly the fundamental facts about mental health 2016 report  by the mental health foundation mentions in a footnote on the 2007 APMS - "Please note that due to reliability issues with the ‘1 in 4’ statistic, ‘1 in 6’ is recommended with regards to reporting figures of people who have experienced common mental health problems in any week".
A lifetime statistic doesn't seem to exist for the UK. but Jamie does write about reports from other countries which reported lifetime prevalence of up to and beyond 50 percent. There is argument however on how to actually diagnose a mental health problem for these statistics taking into account current lifetime circumstances (i.e. where is the line in the mental health problem spectrum of severity)
After a few more minutes searching, I found the 2001 world health report  from the World Health Organisation, and lo and behold on the first line of the press pack fact sheet - "One in every four people, or 25% of individuals, develop one or more mental or behavioral disorders at some stage in life, both in developed and developing countries" . 2001!! a long time before the APMS.
I realised I would be going down a trail of breadcrumbs which may not go anywhere so I stopped there. For me this statistic, even if correct, answers the wrong question.
Whether or not the cause of you feeling in mental anguish is through environmental causes (like bereavement, relationship difficulties etc), or through causes within the mind, mental anguish is mental anguish. I have learnt that there is a way to understand at the very least part of our incredibly complex (but more importantly flexible/plastic) mind and change the way we think. All forms of reflection whether by a diary, counselling or meditation help us to understand ourselves. And just like any other learning process it takes time, energy and purpose. The lows will still be low, the highs still high, but the lows don't have to drag on day after day.
I think mental health problems affect 4 out of 4 people during our lifetimes.
 Horder J. (2010) How true is the one-in-four mental health statistic?
 Bebbington P, Brugha T et al. (2007) Adult psychiatric morbidity in England, 2007 Results of a household survey.
 Mental Health Foundation. (2016). Fundamental Facts About Mental Health 2016.
 World Health Organisation. (2001). The world health report 2001 - Mental Health: New Understanding, New Hope