OP you write so well, your thoughts are so well put together and articulated.
Like you I read everything I could, educated myself in every which way but as you said, it’s the little voice inside you that matters. Ironically I was trying to explain this concept to my child recently and realised I am either such a hypocrite or projecting to make sure he trusts himself more than I have. And just to add beware of too much debate here, it dilutes the voice inside you as well.
That said, I can tell you are looking for some people in the same scenario. ME in capital letters :) Firstly I think as you say, sex is symptomatic of something else, most usually emotional connection. I think being seen, known and on the same page as to what you are doing is crucial to sex, casual or committed. Withholding, distance and a lack of mutual understanding just puts constraints and blunt edges everywhere. Before you do bring this up I suggest you consider what root causes are as this will substantiate what you are saying and he cannot say you are leaving for shallow reasons, which sex is often at surface level perceived to be, particularly given you have a child.
As for my own experience, glacier I have to admit got me at the start. I’ll raise you! It’s fear that has held me back, mainly surrounding the kids. A few other factors at play too. But as otherwise have said you just have to start, I’m sure the moment you say ‘can we talk’, he will know. Stick to your script however and don’t fill in the silence. As another poster said, consider the likely immediate fallout and of course longer term.
It is all about acceptance really, on both sides. I’m not sure anyone fully accepts having to leave a marriage and what that means and the challenges on the other side. Some loneliness, changes to social life, how you perceive yourself as family etc. But acceptance or at least growth on both sides helps things move along. When either or one doesn’t accept it is when it gets sticky but obviously it can’t be forced, just consider it and understand it.
I’m surprised I’ve written so much in way. As the whole process, in my case, as I felt so bad for so long when it was so obvious, just wore me down and thinking about it makes me feel so tired.
I hope this helps. It’s worth it. Be sure you are okay with your own company for a while anyway, that is crucial, you have to rebuild yourself properly after this sort of lack.