Lots to think about, thank you all for your opinions.
To the PP who asked, yes, I knew that he was poly before we started dating properly. Before we even started sleeping together; it took me two whole months from there to say that I wanted a relationship with him and we are now four months in.
It was my choice not to become his primary partner. There is a significant age gap, a signifcant difference in what we both wanted out of a future relationship. He wants children, wanted marriage (but having seen me go through the divorce process may now think differently - other story entirely), wants someone to eventually move in with him. I don't want any of that. He said to me a few times I'm everything he is looking for in a primary partner. We have also discussed the possibility of a primary relationship, but /I/ am the one who does not want that - not because I mistrust him or don't love him enough, but because I know that, in the long run, I would not be happy with the constraints of that much commitment again. Been there, done that, worn the T-shirt many a time.
I don't live day to day from one date to the next. Again, he is far more like that than me; I am, mostly, very happy in my own company. And I am the one putting the brakes on seeing each other too much, partially because I am fully aware that any extra time we see each other at the moment may be taken away once another woman comes into the picture and takes the primary space.
But what some PP have said about me taking a backseat in this is also not true. He operates on a first-come, first-served basis in all aspects of his life, including relationships. He has a hobby that he has been doing for over 15 years, always on the same day. He continues doing that, even if he has visitors, other than full-on emergencies every week, because it came first.
We have talked at length how a potential primary partner would affect our relationship. The answer is, they wouldn't. We would still see each other on the same days we have set aside for each other. He has never, even once, moved those days to make room for someone else. I have been there when he has told friends he can't do X day because that's when I'm around. He plans a lot of his life around those set days, because they are important to him, too. The time commitment is there.
We are an official couple in all capacities. Like I said, his friends, family, colleagues and the woman he will be seeing all know of me and for the first time in my life I did not have to nag him to do so, he did it automatically and out of his own, because he likes to share that we are together.
I know it sounds strange. I have done a fair bit of reading and I know that anxiety and jealousy are a normal part of polyamorous relationships until trust is fully established. And that is it, really, I need to trust that nothing will change. I need to work on my self-esteem and inner security.
That has nothing to do with trying to be 'woke' and everything to do with trying to develop a part of me that has been neglected all my life. Believe me (and I have said that to him, too), I evaluate costs and benefits and if the sum is less than positive I am out. But I also take a long-term view. I am going through a massively shit time in my life and it has knocked me for six, so in the short term I need to bridge my inner issues until they can be fixed.
That is what I need the strategies for. Thank you to everyone who has given me an insight into their experiences, even if it didn't always work out (but then, how many mono relationships do?).