I'm glad you've written that letter for your own sake, but it's too much to send to him: he won't read it all, and he certainly won't digest it all.
I'd cut as follows, to remove most of the feelings and leave in most of the practicalities, because I don't think he gives a shit about your feelings:
I thought I should write you a letter, because I don’t seem to be able to convey to you how I feel verbally, in spite of repeated attempts to get my message across. I don’t know whether it’s that you are too busy to listen, or simply that you do not want to hear. I suspect that, at some level, you have some psychological difficulty in processing anything that is inconvenient for you to recognise, and that this is also why you tune out (even to the point of falling asleep) when you are confronted with anything that requires an emotional response from you.
I am unhappy about the fact that , between us as a couple, the work falls 100% on me. When we are with YOUR parents, YOUR relatives, YOUR friends, I am the only one arranging food and alcohol, cooking, making cakes, booking cottages, arranging restaurants etc. etc. etc. Repeatedly, the pattern between us is that you sign us up for events and house guests who involve a great deal of work, and I do that work. It is completely unequal.
It is especially unequal, and especially unfair in light of the fact that you claim to be are unable to do this work because you are pursuing a glittering career, while I have very little in the way of career success. The two things are even causally related - your decision not to do this labour means that it falls to my lot me, which prevents me from doing other things that would advance my lot in life. This is true not only of events but of a whole amount of domestic labour that is done every day in the house.
But there are psychological effects to drudgery, as well as temporal ones. Being “the help” is not just boring– it is a constant reminder of a state of affairs in which one exists only as a second-class citizen. Other people are too important to get down on hands and knees and scrub floors, clean toilets, or wash underwear - so it becomes one's lot in life. Retaining a sense of dignity and of purpose in the face of this is difficult in a way that I don’t think you can even imagine.
Part of the issue is that some of this structural and economic dynamic is internalised – the work becomes part of one’s essential being, a constant reminder of an unworthiness that is strongly gendered. In a context where I have spent the last six months living quite literally on a building site, that feeling of denigration has been actualised in the shape of a hostile, noisy and constantly difficult environment.
I feel fundamentally deprioritised, too. You have repeatedly found time, money and energy to celebrate your family's events - like your mother’s 70th birthday - but not my 40th. What is more, with finances being as they are, I've not only given up my birthday, but also the chance of a holiday, so you could celebrate with other people. I feel like I just don't matter.
Finally, drudge work that already has more than a hint of degradation about it is made exponentially more so when the people for whom you are doing it who benefit treat you me as incompetent and incapable. I am sick and tired of being bossed around by your parents, and treated like I cannot complete the most simple of tasks without unsolicited help and advice. You and your brother both sit by and do absolutely nothing to intervene with behaviour that is fundamentally unacceptable, even though you both fully recognise that the way they speak to and interact with me and BIL's partner is not OK in private.
Given the sacrifices that I have made, I deserve better than this.
In fact I'd even be tempted to go with:
Given the sacrifices that I have made, I deserve better than this.
because EVERYONE deserves better than this.