I’ll 2nd what hexagon said about going to a library or similar place. Trying to work around other people who are working helps to focus the mind and reduce distractions. If you can go to a library, this may be a good place to set up better practices, e.g. thoses others have suggested, first, then set up a work-only zone at home you can transfer that feeling of focus and work to.
I think the point is, not that we are completely different people, one moment to the next, but that we should embrace change rather than fight it. Life and the world around us is constantly changing and we are better off for changing with it.
Re your hypothetical letter, you can only address it to the persona you last knew, so to speak. Upon renewed communication, you’ll learn how that person has changed, and maybe you will too a little or a lot.