"I don't know how to start"

For some readers everything above is agreed, and the thing that stops them is the first sentence. How do you begin a conversation like that? At what moment? With what words?

The words are simpler than you think, and there's one sentence that works for nearly every worry. "I've been carrying something and I'd like to tell you about it." That's it. It announces that a telling is coming, so the listener can stop what they're doing. It says the thing has weight, so they know not to make a joke. And it asks for nothing yet, so there's no pressure in it. Then you say the thing, in a sentence, as plain as the one you wrote in chapter 1. "I've got a scan result on Thursday." "There's a letter from the bank I can't make work." "Something's wrong between me and Sam and I don't know what."

The moment matters less than you'd think, and more than "never". Not at the door as they're leaving. Not in front of the children. Not at eleven at night when both of you are done. A walk is good, because there's no table to sit across and the silences have somewhere to go. A car is good for the same reason. The kitchen at ten with the washing up done is good. The moment doesn't have to be right. It has to be one where the listener can stop, and you can finish, and neither of you has to be anywhere for an hour.

What comes after the sentence is the part people fear, and it's the part that takes care of itself. The listener will say something, and it'll almost certainly be a question or a "God, since when?", and you'll answer it, and you'll be in the conversation. The dreaded moment is the ten seconds between the opening sentence and their first response, and ten seconds is how long it takes. Chapter 21 goes through the whole shape, but the shape is mostly this. One sentence to open, one to say the thing, and then answer their questions until you've said all of it.

If you can't say it, write it. A text that says "I've been carrying something and I'd like to tell you about it, can we talk tonight?" is a telling that's started, and the hardest part is done before you've seen their face.