As I watched the 20-year celebrations of the Good Friday agreement play out, my frustration and anger began to boil over. “Where the fuck is she?” I wanted to shout at the television and radio. As the day went on I decided to just avoid the media altogether; it was only going to make me cross, and that’s not what the day was about. It was a day to celebrate something extraordinary, the impossible becoming possible.

Mo Mowlam was my stepmum, but for most people she is remembered for the work she did helping to bring peace to Northern Ireland. When Mo arrived as Northern Ireland secretary in 1997, the job was reputed to be a “poisoned chalice”. However, Mo not only brought the opposing parties to the table but also helped persuade a country of divided communities to come together for the sake of peace and progress.

I missed Mo more this week than I do on the anniversary of her death, or her birthday. Her absence was everywhere in the British media’s coverage of the anniversary of the agreement. I felt powerless as I watched the old guard being paraded across our screens. In their defence, Mo can’t be there of course. She died from cancer back in 2005.

On Tuesday, Tony Blair gave his speech in Belfast on the anniversary of the agreement signing. It was a stark oversight that he didn’t even mention her name. He made no acknowledgment of her role in the negotiations at all. Perhaps he was afraid that he might get another standing ovation about someone else in the middle of one of his speeches?

I would go so far as to say that without Mo there would be no peace process in Northern Ireland. She applied her own distinctive personality and approach to the job. She demonstrated empathy and understanding. She recognised and listened to victims and survivors. She worked closely with the men but also very importantly the women – mothers, daughters, girlfriends and wives – of Northern Ireland in order to achieve peace.

Mo Mowlam visits Belfast, four hours after being made secretary of state. Photograph: Paul Mcerlane/AP

Mo was from a family of strong women; her father was an alcoholic which meant her mother and older sister held her family together. Her inclusion of women was imperative to the peace process – she knew that without help from the women in the community, there was no way she was going to be able to persuade the men to join the conversation. She had to find the humanity in and commonality between families and relationships from all sides, in order to help people find hope.

Some have argued that Mo was not able to actually “seal the deal” in the end, and that things began to unravel on her watch. Or that there’s a “myth of Mo” and her role in the peace process. In 2005 when Mo died, Blair said of her: “It is no exaggeration to say she transformed the politics not just of Northern Ireland itself but crucially of relations between the Republic of Ireland and the United Kingdom, and it was this transformation that created the culture in which peace-making could flourish … suddenly nationalist, republican and Catholic Ireland had every preconception of the English up-ended and rendered out of date. She didn’t have to talk about equality. She exuded it, naturally and with an absence of affectation that was marvellous to behold … she bowled everyone over.”

The thing about Mo was it came to her instinctively. She didn’t think about it, it wasn’t a strategy, it was passion

As a director of documentary films, I’ve spent a lot of time in Northern Ireland and the republic since Mo died. The island is a second home to me, having been so formative in my teenage years and early 20s. In anticipation of the rewriting of the history books (or at least the omission of certain characters!) I’ve been pitching a film to commissioners at various broadcasters: an authored, personal story in Mo’s memory that would celebrate and explore her legacy for contemporary women. I’ve been told “no one would be interested”. That they “couldn’t see who would watch it”.

Seemingly a fitting narrative when you look at what played out on the media over the past few days.

The only place I have seen Mo remembered as she should be this week was in the middle of a documentary presented by the comedian Patrick Kielty in which he recalled her honesty and straight talking. It made me smile watching the archive of a show he had hosted, with her as a guest. As he said, Mo was “greeted like a rock star, not a politician” when she entered the TV studio. A testament to her humanity and ability to connect with all walks of life.

Everyone has a story about Mo, and I am constantly amazed by how many people she met in the world. How many people she touched. You can’t just erase that memory, however hard they try. She never just “flew in when the cameras were turned on” – if anything she snuck out and went to see people when no one was watching. The thing about Mo was that it came to her instinctively. She didn’t think about it, it wasn’t a strategy, it was instinct, it was passion and it was sheer determination.

Mo knew the Good Friday agreement was just the beginning, she knew it was a process and when she died she hoped that whoever took over those reins would honour that, would work at that and would help the people of Northern Ireland to progress to true peace. It wasn’t the finished article, and this is where I believe people are going wrong. They are celebrating an end, a solution; but it was a beginning, a starting point. Neil Kinnock at her memorial started his speech about Mo with a very simple statement: “A light has gone out.”

I often think of those words. A light did go out when Mo left the stage, and while many people will attempt to live in the afterglow of her memory, they cannot remove her from the memories of the people.

• Henrietta Norton is a documentary director and writer. She is the director of the feature film Born And Reared, and has made short films for the Guardian and the New York Times