Thank you everyone, this is sobering for me. I felt a little shell-shocked returning to this thread this morning. 'The water in which we swim...' and all that.
I'm public sector (naming the profession feels a bit outing, but it has been mentioned) so there isn't really any flex in remuneration structures -we are all working on a set scale, and there is no money, nor inclination within the organisation, to move or negotiate on pay. I have been offered an increase in management responsibility in exchange for a small incremental pay increase, but I had to turn it down as I knew that I wouldn't be able to manage what would have been a big uptick in hours.
A PP suggested I might have ADHD and not be managing my workload well -this hit home, not because I actually think I have ADHD but because the workload comprises such a variety of day-to-day tasks, all of high priority (if they're not completed, the following day simply can't happen), and the level of scrutiny and high-stakes expectation ('aspiration' in the workplace parlance) makes a sort of high-grain level of micro-management within my team and the people for whose progress and performance I am responsible, almost inevitable. I've codified and structured everything to within an inch of its life, but at the end of the day we provide a bespoke offer and as such, there are many processes that can't be replicated or repeated, but have to be reinvented daily. A very 'broad' focus on many moving parts is required, as opposed to attention being directed to a smaller number of projects in a linear fashion. I do feel like I'm drowning quite a lot of the time. It bugs me that -despite having a pretty bogstandard neurotypical brain and no processing issues to speak of- ADHD seems like a plausible explanation.
I daren't do the £/ph calculation. We need my income as I am the higher earner, so even dropping hours or changing careers with an initial drop in salary would be financially perilous. I have considered retraining.
A PP suggested banking on the assumption that I would not be sacked for holding a firmer boundary, but in the last few years I have seen as many colleagues being 'managed out' in response to raising concerns over workload. There's supposedly a recruitment and retention crisis in my field, but it seems to be an open secret that capability procedures and support plans follow efforts to dial down the expectations on out of hours working. NDAs go hand in hand with resignations and dismissal in these cases, so there's a really uncomfortable social aspect to it that I'd really rather not risk.
On the upside, there are periods in the cycle when it is not this busy, and periods when working entirely from home is possible.
Not being straight with DP does bother me; he is the most honest, upfront person. But although he's also worked in a demanding public sector role, it never involved any expectation of additional hours beyond the contracted ones so he doesn't really get that although my contract states xxxx h/pa, it also specifies that I am expected to work as many additional hours as it takes to fulfil my role -I think this is pretty standard, though.
Thanks again, I have read and reflected on every comment. I'm off to browse CS roles, a bit of a holy grail in my field.