Due to become HoD next month. Dreading it.

I was very inexperienced when I joined this department. I had only ever been a GTA (which is very different from being a member of staff), and I had worked for a bit in a distance learning context which was very different to how most university department works. For example, when I started teaching here I wasn't even made aware that I could ask to have GTAs - I did see that some courses did have GTAs, but then there was a whole discourse of "oh we have so little money we're on the verge of financial collapse!" and so I thought I'd rather not eat into the budget if I could do things myself. With two HoDs, in particular, I realized at some point that they didn't even have a notion of how much I was teaching, they just kept piling up stuff on me but not keeping track of it, they kept forgetting I was teaching X or Y. Generally speaking there is very little clarity in my institution and particularly in my department about a lot of things, for example, we don't even have a workload model in my department (this is being introduced from next year), under the premise that "we don't need one because we're all reasonable people and we can work out things among ourselves", so at some points I could see that, even on a research-only contract, I was teaching more than others on a R&T contract... but then there was no written or spoken rule in the workload model that "R-only people must teach less than R&T people" (although you could argue that it is kind of obvious from the name), it's all "to each according to their abilities" or whatever...

So to answer your question. In my first three years I was only on a temporary contract, there was the promise of it becoming permanent afterward, so I figured out that being outspoken and "difficult" wouldn't earn me bonus points towards a permanent contract. There's of course youth (not anymore now haha), naivity and stupidity. I think me starting to be outspoken now would be tantamount, in the eyes of my colleagues, to a "Dr Jekyll to Mr Hyde" transformation.

in the short term maybe identify some specific red lines (such as not doing the HoD job without X, Y and Z) and ask for a meeting with the right people to insist on these

Yes. I know what you mean re the chances of solving anything through a complaint or grievance process. I think what I want the most is an acknowledgement that I was treated wrongly, which I know is childish, almost ridiculous to go through a very difficult process just to get an "I am sorry". And re "red lines" - I am now realizing that when I chickened out about the role, I let myself be talked into it again on the basis of very vague promises. For example there was talk of enrolling me in a Leadership programme and there's been zero movement on this ever since, when I am pretty sure that recruitment will be happening now if it hasn't closed already for university's Leadership programmes that start in September. My DH is also of the opinion that I should set specific conditions and if I'm not given them then I just walk away (and at the same time keep looking for jobs), this is what he did when he was in a similar situation years ago (also in academia but different uni) and he was successful.

The coach suggestions are also very good, I think I'd rather do this privately as I'm not sure I'd feel comfortable talking to someone within my uni (even if they're a completely different subject) but this might pay off for itself if it results in an earlier promotion.

Also I am of course not happy that many of you have gone through similar situations. But they also bring some reassurance that I am not imagining things and that this isn't happening to me simply because I'm stupid and weak, rather there are more profound gender issues. I'm in an almost all-male department where everyone prides themselves on being oh so feminist and civilized, academia can be the most hypocritic place sometimes.

There's also a history of colleagues overriding my authority with students. The latest is that I have been convenor of a programme for a few months, I rejected an applicant (after giving them a chance to re-submit their application), the applicant somehow found my predecessor's e-mail in an old course brochure, e-mailed him, and my predecessor e-mailed Admissions to tell them that the student should be given an offer. When I raised this with upper management they said "don't worry we'll always back you up and never undermine you", but yeah... who will you back up in the hour of need - the professor you've been pals with for 20 years or the lowly SL who has been at the uni only for half of that time, kept mostly a low profile, and never been part of the in crowd? Even if I end up doing this, I have no idea how I will be taken seriously.

I'll be a Reader by the time I start (what I didn't get was promotion to Professor)... I'll be eaten alive if I rock the boat in the slightest.

I think it's a "damned if you do damned if you don't situation". Last month I plucked up some courage to talk to a colleague who was suggesting I give up the HoD's office to build some kind of teaching facility (which he says will bring international students in huge numbers) and I move to a much smaller office... I said that this is a rather radical move that would also require us to redistribute everyone else's office space and I would feel uncomfortable taking any decisions without a proper market study, and he didn't snap back, he just said "of course" but I think he was genuinely shocked that I wasn't immediately going along with it or at least showing more enthusiasm. But even saying that, which is objectively very non-confrontational, took a huge effort and soul-searching - I was thinking about how to phrase it for about 3 days, I cannot spend hours thinking how to phrase every sentence I utter.
I feel somewhat more stable and optimistic today (had a really long night's sleep last night), but it's not great as it also makes me think "well this isn't the end of the world and if you face a few shitty situations during the next three years and don't get promoted to professor until after that then so what, there's much worse, some people your age haven't even got their first permanent appointment and your have some recognition for your research outside the uni".
DH and I are reuniting tomorrow (I started holidays early while he was on a work trip) and we've agreed to work together on a plan. I've also got an executive coach recommendation (someone with a background in academia) and will e-mail them to see what they can offer.

The last 24 hours my anxiety has skyrocketed. My husband arrived, we gave ourselves until the end of this week to plan how am I going to articulate my conditions to my line managers and how. However last night I couldn't sleep at all, could hardly eat anything today, late afternoon was going a bit better until a couple of hours ago where I've been crying almost non-stop. There's this colleague at work that I thought I could confide in, and I thought it would be a good idea to seek him as a sort of ally, in the sense that I feel he's always valued both my research and my hard work, and he would be sympathetic and helpful if I told him I've been feeling undervalued and taken for granted.
Today he e-mailed about something else but because of the tone and nature of his e-mail I thought there was an opportunity there for me to voice some of my concerns about being undervalued, which I did. His reply was that I am wrong and that university systems for recognition and workload are very rigorous so the only possibility is that I am wrong. He doesn't even say "I'm sorry you feel like that, but I think you are wrong", which I would have respected much more.
What a disappointment. I feel that, regardless of what I negotiate with my line managers, at this point the only way is out.

Honestly, why not get signed off for a couple of weeks

This is looking increasingly likely now. Looking at my e-mails (which I shouldn't since I'm on annual leave, theoretically) and reading about specific tasks I'll need to do for next month often sends me into a spiral of anxiety and panic that can take half an hour to one hour to subside. Clearly I won't be able to get much done if this continues.

But not women THEY KNOW and not perpetrated by THEIR FRIENDS.
This is something I'm learning the hard way, and I'm angry at myself that it took so long. Academia is horrible and toxic; but not us, not our little group, we are all friendly, supportive and wonderful.