There’s a lot of talk about trust here. The comments of all those who are horrified at OP ‘s husband reading her diary, would be more than justified if it was a child, parent or sibling, but although their feelings about a spouse being entitled to privacy are justified, on the other hand one could argue that if a couple are happily married, they would share and talk through any problems and not keep secrets from each other. I can understand that a husband might feel disappointed and left out, on reading the diary and discovering that he was not being treated like a helpmate and confidant, but I also understand the motive of the OP in keeping him in the dark and now feeling betrayed.
But I do think it was naive not to hide the diary and maybe the husband saw that as an indication that it was not really private, at least in relation to him.
Perhaps the attitude to keeping a diary is different for women - they are genuinely secret thoughts, but for men the motive might be quite different - they really want them to be read?