Northern Ireland has long been viewed as a “place apart” within the United Kingdom. Now, unionists fear it is about to become a “place apart” outside the UK: separated from Great Britain by a new border in the Irish Sea, pushed closer to the EU and, eventually, into a united Ireland. That’s why they are unsettled right now, and indeed fearful.

Unionists have been here before. In 1972 their parliament at Stormont was prorogued, and direct rule imposed from Westminster. In 1973 the British and Irish governments, with limited input from unionists, concluded the Sunningdale agreement, which replaced majority rule with mandatory power-sharing and an “Irish dimension”. A little over a decade later the British and Irish governments signed the Anglo-Irish agreement, which unionism regarded as a form of joint sovereignty. And in 1993 the Downing Street declaration stated the British government has no “selfish strategic or economic interest” in Northern Ireland.

Each of these moments represented an enormous political and psychological blow to unionism. Each was accompanied by huge anger and enormous rallies. Yet while there was a fear that unionism was being pushed in a direction it didn’t want to go, Northern Ireland was still an integral and equal part of the UK. Similarly, while sections of unionism, including the DUP, initially rejected the Good Friday agreement in 1998, it was still possible to sustain the argument that Northern Ireland would remain fully within the UK until a majority of its citizens decided otherwise.

There is an anger across all sections of unionism unseen since 1985 and which may prove very difficult to contain

Boris Johnson’s Brexit deal is very different. If it emerges from the parliamentary debate and scrutiny stages with its Irish sections intact then Northern Ireland will cease to be a full and equal part of the UK. Yes, Johnson could still argue that it would require victory at a border poll for Northern Ireland to formally cease to be part of the UK, but unionists believe that beginning the process of easing the region out – as his deal does – makes it much easier for Irish nationalists to make their case for reunification.

This is a hammer blow for the DUP. It has propped up Conservative governments since June 2017 and its 10 votes have saved Theresa May and Boris Johnson on several occasions. The unionists, not surprisingly, believe they are “owed” for that confidence and expected Northern Ireland’s constitutional status to be protected come what may. May let them down with the notorious backstop. And now Johnson has let them down. Even the hardline Brexiters of the European Research Group have abandoned the DUP. What makes it immeasurably worse for Arlene Foster’s party is that it made such a song and dance about the influence it had with its friends in high places.

It is a bigger hammer blow for the rest of unionism, raising the extraordinarily awkward question: if the DUP has been “betrayed” by the party it had kept in power (and whose key players, including Johnson, had come to Belfast to trumpet their loyalty to Ulster unionism) then whom can unionists now trust to protect their constitutional interests?

So far, that question remains unanswered. Which is why some elements of unionism/loyalism (and while they remain small in number, they have a worrying potential to grow) seem determined to send “some sort of message” to Johnson, the Irish government and the EU. But it is very difficult to send a strong message without damaging their own argument; because it is their own sovereign parliament, to which they are so loyally attached, that is ultimately responsible for their predicament. Put bluntly, it’s very difficult to bully your own national government into treating you “equally” when both your own government and, the evidence suggests, an overwhelming majority of your fellow citizens across Great Britain, seem to have no particular interest in sustaining a link with you.

It was very noticeable at the party conference on Saturday how much time during speeches and at a fringe event was devoted to the importance of the union and the lengths the DUP would be prepared to go to shore it up and maximise support for it. Indeed, this focus on the union was the clearest possible sign that the party recognises the dilemma unionism is currently facing.

There is today an anger across all sections of unionism which I haven’t seen since 1985 (one mass rally at the time sparked DUP leader Ian Paisley’s famous “Never! Never! Never!” speech). And, because Johnson’s deal brings the end of the union closer than has any previous pact, it is an anger which may prove very difficult to contain. Which leaves the DUP with some problems: how does it contain the anger, restore its credibility within unionism, block Johnson’s deal, and lessen the chances of an early poll on a united Ireland?

One thing it now knows for certain is that it cannot trust Johnson: even if the prime minister tries to appease it with a lorry-load of reassurances. The party’s priority must be to find a way of removing itself from the series of hooks it has placed itself on since June 2016, when it backed Brexit. And it will have to admit that the uber-unionist nonsense that accompanied its relationship with the ERG was a monumental miscalculation. For the DUP, as with all Northern Ireland unionist parties, the union eclipses every other issue.

I suspect the DUP knows all this. Which means it also knows what it needs to do. There is no enthusiasm for Johnson’s deal in Northern Ireland (the nationalists are hardly fans either – they strongly voted remain). Unionists will never agree to anything that changes the constitutional position so significantly. All of which can ultimately lead to only one conclusion: the DUP must now shift towards remain. It will have to – for its very survival.

• Alex Kane is a former director of communications for the Ulster Unionist party