What is it like living with Alastair Campbell? I get asked this a lot and the answer, not entirely in jest, is usually: “Bloody difficult… but never boring”.

From the moment we met almost 40 years ago as young journalists on the Mirror Group’s graduate training scheme, it was clear that Alastair was a complex, charismatic and challenging person.

Fiercely intelligent, exceptionally driven, he had a very effective ability and taste for commanding the space around him and getting what he wanted. We started going out together quickly and, partly through circumstance, moved in together after two months.

It was the start of a relationship that would last decades, give us three wonderful children, provide many highs but also some anguished lows, largely due to Alastair’s undiagnosed depression and subsequent mental health issues.

The drinking in those early days ought to have provided some clues. Much later on, I came to understand how people use substances to manage depression and anxiety. I also eventually realised the complex nature of addictive personalities and the impact of their behaviour on others. But back then the frantic socialising was part and parcel of our rapid progress to jobs on national newspapers before we were 25. Fleet Street revolved around the pub, and nearly everyone drank to what we now know is excess.

A change of job in the mid-1980s followed by a period of intense stress saw Alastair’s drinking reach epidemic proportions. His behaviour became erratic and aggressive. I remember at one point my Scottish father taking me to one side and saying: “Are you sure you want to stay with him?” Even though the answer was an emphatic yes – I didn’t really have any doubt – within weeks we had temporarily split up. Alastair moved into a hotel and radio silence descended between us until I got a 1am call to say he’d been arrested and was in a police cell in Scotland.

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The psychotic breakdown is explained in his new film, to be broadcast this week on BBC2. Once Alastair had come through the worst of the crisis, after a short period on anti-depressants and having convinced himself that drink was the cause of his breakdown, he quit the booze, shunned psychiatrists, resolved that he could cure himself and achieve his ambitions.

Initially that appeared to be successful. He went on to be political editor of the Daily Mirror, was pivotal in the election of New Labour and commanded the political heights as Tony Blair’s Director of Communications in Downing Street, where we both worked for six years. Only much later did I realise he did all this as a high-functioning depressive and was probably ill throughout much of that period.

The workaholism and demonic energy, the mind-numbing crashes that would sometimes leave him unable to get out of bed at weekends, though I bitterly noticed that he would rouse himself to get to work on weekdays, were all signs we misread. His moods are extreme, as he admits in the film. He measures them on a scale of one to ten – one ecstatic, ten suicidal. When he sets his mind on an objective he cannot be budged from it; you can see why presidents and prime ministers would want him on their side.

He is also loyal, funny, brilliant in a crisis and can be very kind, especially to friends in need, but his behaviour can tip into something more dangerous. There was emotional manipulation and mental cruelty at times. The fact that he refused to speak to me for at least three days on a holiday with three young children really wasn’t funny and marked a toxic aspect of our usually happy life together. Only now do I realise his depressions could change his personality.

Then there were my own feelings of guilt and failure for being unable to make him happy at times like this. I have been sad and experienced grief but I have never known depression. How could he get up and function normally at work, be affectionate and engaged with the children, but reserve his silent, black dog moods for me? I still don’t understand this, and he can’t explain it other than to say he was sending a signal that he wanted me to be there while not wanting me to be there. I vented a lot of my anger and misery by pounding up and down the lanes of my local swimming pool.

Not enough is said about the partners of people who are mentally ill but when I have spoken publicly about our life together, I have always been inundated with messages from people in the same situation, grateful that I have articulated their own experiences. After we left Downing Street, and there was more time to reflect without the distraction of a high-octane day job, the crashes became longer and more frequent. A close friend urged Alastair to see a psychiatrist, who helped both of us understand more about his illness and led Alastair to accept the need for regular medication. It also helped me to understand my own emotions, the part I may have played in the difficult times and how to ride out the low points, and not to see his depression as being about me.

I know people who feel they couldn’t stay with partners whose serious mental health issues threatened their own wellbeing. I can honestly say that I have never seriously considered leaving. My default position has always been to want to help Alastair get better, even if that often has seemed pointless. Thanks to good psychiatric support, the right medication and increased self-awareness on both our parts, I would say our lives are better than ever.

I hope the growing awareness of mental illness, the willingness of high-profile public figures to talk about their struggles and the media coverage will help other people. When Alastair was heading towards his nervous breakdown 30 years ago I attempted to raise his mental health with his then boss but was brushed aside. I hope and believe that wouldn’t happen as readily now.

These are challenging times for people seeking psychiatric support on the NHS, but if I did get the chance to advise my 22-year-old self, I would say get professional help as soon as possible, find something you can do for yourself to relieve the stress, and avoid blame. With the right support, mental health conditions can be managed and alleviated. It will never be plain sailing but I hope that our experience gives others courage and the confidence that they can come through.

Alastair Campbell: Depression and Me will air on BBC2 on Tuesday 21st May at 9pm.