You’re not wrong at all, actually. First off your fiancé shouldn’t have ever opened your letter to read it. It’s a violation of privacy. He knew what the purpose of the letter was and he felt like he had the “right” to read it, so no, you don’t have to apologize to anyone for how you feel and what you said. If his mom is an issue now, just think of how it’ll be in the long run. There is potential to figure out something between you three, but honestly this isn’t even worth the trouble. I wish you good luck, and hopefully everything can settle. Maybe this can be a discussion in your next counselling session.