I am 40 this year, unmarried mother of 4.
My perspective is a little different.
Marriage was a concept dreamt up thousands of years ago to ensure that daughters(otherwise unable to take care of themselves) were taken care of.
It was a business arrangement, a family was found the man offered a dowry in exchange for taking care of the woman.
This have fathers/families some recourse if husbands broke their end of the bargain.
It was a. Convenient arrangement amongst families, and was essentially good for society.
It was also created at a time when the life expectancy was around 35 and women were married off in their teens and often died in child birth.
It was never meant to last a lifetime, in fact I am not convinced human relationships are meant to last indefinitely.
Thing is relationships, like the people in them are living, breathing, growing, changing things.

Humans are forever in transition, growing, learning adapting.
What I wanted in a partner when I was 17, isn't what I wanted or needed at 25'. I would say that Chamberlain in my 30's.
In fact I spent the best part of a decade out of relationships because I just didn't know what I wanted or needed.
It's only In the last couple of years I am comfortable enough on my own to be in a relationship that feels like a shared experience rather than a dependency of some sort.
Of course this is simply one person's perspective.

What I am saying is I think relationships are something that cause us to grow and change, and not always together.
Those snap shots you see on fb, are usually the best seconds of an ever changing and fluid partnership. Many of which never make it further than 5 years once the marriage certificate is signed.
You are no less of a person your experiences are no less worthwhile just because you haven't been married.

I hope your able to find what it is you feel is missing. Take care and all the best.