It takes a long time to condition someone. You've been conditioned to feel ashamed (and secretive) about your poly-ish relationships (such as the one with M2). Conditioning like that isn't undone overnight, so have some patience with yourself as you try to undo some of the old secretive habits and open up to M1 a little more. You have to learn to trust people to accept you for who you are. You haven't been able to trust that in the past (with your previous husband), and trust is only built up little by little. So know that even though things feel very odd now, they will gradually get easier.
Journaling is a great idea. We even have a Life stories and blogs board
on this site if that would help.
Just know that we're pulling for you here, and M is pulling for you at home.