You're a Sounding Board, Too
You know how you have one friend who's a good shopping partner, another who likes to go out and drink margaritas, and one who's your go-to workout buddy? Guys like a less complicated support system -- why not look for one person to meet all those needs? And perhaps your fiancé's mother is his go-to confidante for everything from work-related issues to his personal insecurities.
He's not slighting you on purpose; it's just that a good mother knows how to make everything all better. Remind him that you want to be the one he comes to with his problems, particularly those that impact your day-to-day lives. If he's sensitive enough, he'll probably suspect that you're feeling left out, and he'll be more inclined to let you into the fold the next time he has something to talk about.