But it freaks me out a little bit knowing that I am going to live the rest of my life with no mystery, no adventure, nothing new.
It's not about wanting to be with anyone else, it's about growing on my own.
We have nothing to talk about, ever.
I would heartily disagree with these statements. Not that this isn't the current state of things, but that relationships don't HAVE to be that way. Ever. It's totally what you and your partner make of it.
I've been with my husband for almost 5 years now, married for 2, and I can honestly say while I was so worried about those things prior to meeting my partner, I found out we do a pretty damn good job of naturally keeping things interesting, fresh and fun. It does take work though - I have learned to communicate and start coming up with new ideas of fun things to try, places to explore, etc.
And now I find a depth in the familiar; the complexity and layers of our relationship are so much more exciting and fascinating to me than the excitement of something (or someone) totally new.
Now I did notice based on your post you have been with him since you were 18, so there's probably a lot more pent up "I want to live my own life and explore" than there would be for someone in my position, so I can totally understand that. And you also have motherhood on your plate as well which would definitely add in more life responsibilities to add to the partnership plate.
I talked to him about it YEARS ago but aside from that he has no idea. I am afraid of stirring shit up.
Talk with him. Talk with him about everything you spoke about above, show him this post - hopefully he will be encouraging and supportive and let you try out new things on your own without having to leave the relationship entirely or your home with your current family. Sure, get through the holidays, but afterwards make it top priority to tell him how you're feeling and struggling.
You two can work on solutions together that don't have to involve you just up and leaving the relationship. I definitely find in my relationship I have ample opportunity to explore individual things for myself that don't involve my husband. It doesn't mean I cut him out, but I do get to go do my own adventures and then can come back after it's said and done and chat with him about my experiences, struggles, victories, etc.