Just my opinion, but I wouldn't send it. You apologize and I don't think you should. You talk about making changes in yourself, but you shouldn't tell him that, just show him by your actions.
I think at some point he will have to be 100% transparent or it won't work. Can't save a marriage with 3 in it. Maybe though that is something you can discuss in MC? I think he wouldn't feel the need to maintain privacy unless there is something he doesn't want you to know, i.e. an EA or PA or about to be one. So eventually that will be a problem. If not now, then discuss with your MC?
