One thing that helped me when I was in this situation was setting myself a deadline by which I'd either make my peace with the situation, or leave.
We moved in together after a year together, and got engaged after 2 years together, completely unprompted by me, so I had no reason to doubt a wedding wouldn't be forthcoming.
My DP at the time mentioned getting one event out of the way, then we'd start planning the wedding. With that assurance, we started a family, and had 2 DC in the following 3 years. I then waited a further year before asking about the wedding, when alarm bells started ringing as he didn't seem that keen, and other than the proposal had never brought it up again.
By then we were 5 years into the relationship, and 3 years engaged. With the babies and a busy life, it was a further year before I got round to querying about the wedding again, and got the same response. I resolved to give it another year, so my deadline was 5 years engaged.
It helped enormously that my DP started taking me for granted, and became emotionally abusive in the last year or so, and I resented being put in a position I really hadn't planned to be in, so it was by then a far from happy relationship, which made it much easier for me to leave.