I personally would not write the email. Write it in a journal for yourself, to remind yourself. But would there be anything in there that you have not said before in some way, shape or form?
I used to think that if I just explained, then he would get it and pay attention to what I was saying. But words, explanations were a hook, they simply gave him another way in. I told him things which he could later use to get a response or turn around on me (even to the extent of bringing private correspondence sent in good faith into court to show how unstable I was - which the judge ignored; of course the point was to upset me, not to get a reaction from the judge). My point is that your purpose is to try and reach a common understanding but his will be to find ammunition. An email with everything you feel simply makes you vulnerable; he is not stupid, and surely in twenty years you have tried to explain. Nor is he spoilt by you (more victim-blaming!), rather he has worked out over the years how to get what he wants by emotional manipulation, threats and tantrums.
Regarding the phone call, it is trying to rein you in and get you back, yes. There is not a common conversation which also addresses the situation of your mother and your need to support her.
I think, hard as it is, you need to think very clearly and in a focused way, about what you want to do. I feel that it goes beyond my capability to advise on the situation as regards your mother, but maybe post on the Living Overseas forum. I might be wrong but I think there is also an Elderly Parents forum on here too. What are the decisions you need to make regarding your mother and what resources do you need to make them? What are the realistic options? I suppose that is the first and most important thing.
How much time do you have off work?
In my situation, with the whole long, running legal proceedings and court case I had two clear objectives regarding DC. I will not put them here but they were quite reasonable. Everything else he tried to draw me into was smoke and mirrors. My point is that you do need a clear idea of what you want to achieve here and then it will be easier not to be deflected by the manipulation and attempts to control. To some extent, your partner does have some reasonable concerns about income and the future, so you need to also have in your head your position on that (acknowledgement of the situation, a clear idea of what you are doing about your job and business and when - and you are right, going from occasional sales to main breadwinner is not a reasonable expectation) and where it fits with what you want to do.
At this point, given the pressures on you, I would be tempted to see writing any emails explaining how you feel as a side-track and potential diversion (just as I see a debate about whether he is spoilt or ego-centric as a diversion, the end result is the same; the only point is that you have not spoilt him!). Focus is needed, and this will make you anxious as you are not used to putting your needs first. Focus and clarity about what those needs are.
Hopefully that makes some sense.