I totally empathise @ApFlack .
I'm in a similar situation and have similar feelings.
I've been in several very long term, good quality cohabiting relationships, good career, nice life, friends, hobbies etc. Never married by choice, child free by choice. And previous partners have been in the same situation as me, so it never arose before.
Current partner was married (she cheated & left), then had a child in a subsequent relationship (child's mother left, just general incompatibility, they co-parent well).
I will never be a wife (would be a terrible idea for me financially), and will never be a mother. I never wanted either of those things, but even though my opinions haven't changed, I do really feel somehow lesser in this relationship.
Maybe it's something about there being things he's shared with other people that we will never share, a sense of there being an imbalance between us in terms of certain life milestones and key events, even though I never wanted them...
He's lovely about it, does his best to be reassuring, but doesn't really understand why I feel this way. I don't either. But despite how lovely he is, I quite often daydream about leaving to be with someone who has similar life experience to me, simply because I find it a problem, and find it hard to visualise a future with him long-term (& we've been together 4yrs, living together for 2).
I wonder how it feeds in with your relationship with his children as well?
I like his child, I think it's mutual, and we get on fine, but I don't feel like we're a family unit, there just isn't that depth of connection or sense of unity there that I experience in a family, and because obviously his child is his priority, it does leave me as the outsider quite often, whether that's reality or only in my head...
No real solutions, but you're not alone.
Watching with interest, maybe someone has a different approach to consider!