i found myself in a very similar situation about 3 years ago. i had a bf of about 5 years - we started dating when i was 18 and he was 25 (i know, i know). we had gotten to the point where i was ready for the next step after college and i just started asking questions about our future together and which resulted in fights. then he unfortunately got very sick, which put a hold on things. after he was given a clean bill of health, i brought up the conversation again in a very light way several months after he was totally okay (i was trying to be sensitive for what we had just gone through).
however, after getting so much static (and i mean it was BAD), i realized that if this experience didn't change his outlook, nothing i was going to do or say was ever goign to get him to commit. until i told him i was leaving -- and i meant it. i did exactly what AOTB had suggested on other posts with the mental deadline. then it seemed he could only say exactly what i had been waiting to hear for about 2 years in all. it was pretty weird how quickly those tables turned, and i realized how not in control of my own life i had been -- i had been living my entire life for him and had compeletely lost my sense of self all the whilte. walking away from him was the BEST decision i have ever made in my entire life. at that moment, i took control of my life and decided to put my happiness first.
now, i'm not saying this is what happens in every case. personally i would not want to marry someone that i have to "push" into doing anything, especially marriage. again, that's me and i respect and understand that not every woman feels this way. i just find it sad that too many women are in this kind of situation: she's a fantastic woman, ready to commit; but for whatever reason, he just isn't on the same page.
to the OP: i seriously hope that you can work through exactly what it is you want and i sincerley wish you the best of luck. it's a tough spot to be in.