7 years no engagement

So I'm getting really irritated. I've been with my bf for 7 years have had 3 kids 1 on the way and I still haven't been proposed to. He has been married before and asked her to marry him within months and married her within months. Why not me?? Why after 7 years he can't even make that commitment? But he was able to ask her and marry her within months?? I bring it up to him and it's like he doesn't want to hear it. He has no plans on asking nor do I think he ever will. What was so good about her that he would marry her and not me! I'm so frustrated and in tears about it cuz I feel like I'm not good enough to marry. I don't know what to do or how long I can be in a relationship that doesn't seem to be heading where it should. It's not fair cuz I'm not content and he is. Ugh!!!! 😡