I'm sorry to hear this OP, when I read your first post I thought are you talking about me???
I'm in the process of leaving my 'D'P after the same amount of time, and same ages of children, as I finally came to my senses, and realised he's been future faking me all along.
Within our first few dates we discussed marriage down the line (he'd been married and had a DC before), as you do when working out whether it's a fling, or more long term.
At the end of our first year together, we moved in, and at the end of the second year he proposed. When discussing dates, he said he couldn't do it for a couple of years due to his immigration issues, so we should try for children in the meantime, and he promised we'd get married immediately after he became a citizen (we were both late 30's by then).
We got pregnant the month immediately following the engagement, bought a house the next year, then had asecond DC exactly a year after the first.
Now, whenever I bring up the subject of marriage, there's a different excuse every year. Once he got his citizenship (organised through work) he then said we had too much debt, so I paid it off. His next excuse was that we didn't have enough saved, so I saved up. His next excuse was that my sister and sister-in-law were due babies around the time I wanted to set the date for. Next excuse was that we should get married on our 10 year anniversary, and when I said no, and suggested a date next year, he said he couldn't get married because his mum will be on holiday on the date I wanted to book, and he's not paying for her to come back 2 days early 
I finally admitted defeat when he claimed he's now not sure about marrying me because he feels like I don't respect him, and he needs to be in a place where he feels I do. Absolutely not true on my part, and it's just another way to kick the marriage can down the line, and leaves him in full control of deciding when HE wants to do it (or not). I'm worth more than that treatment.
I'm currently getting all my ducks in a row, and will tell him it's over by Christmas. I'm in a very good financial position though, as I earn more than double what he does, we have a decent amount of equity in the house to split when it's sold, and I have a wonderful support network. After all this being messed about, I don't even want to marry him anymore!
I'm telling you my story OP so you open your eyes. He KNOWS you want to get married, he's TOLD you it will happen, yet he still does nothing about it. His actions are telling you everything you need to know unfortunately, you need to listen to what his actions are saying. Good luck with deciding what comes next, I wish you all the best.