Usually when a couple is pretty serious, they discuss the idea of marriage for quite awhile before any proposal happens, more as a hypothetical thing or even just in a joking way, it lets you get a good idea on where you're both at if you both seem comfortable with the idea.
I'll be honest and say that it sounds like perhaps you got a little ahead of yourself, it happens sometimes, there are far worse mistakes to make in a relationship than that.
Maybe if you're not sure how to talk to her about it send her a message when you're a little more clear headed (DEFINITELY not before) and you know, just let her know that you realize maybe you jumped the gun a little bit and just let her know how much you care about her, and that if she's not ready for that kind of life then she doesn't need to be, she's allowed to have all the time in the world to be ready for something like that, two people who care about each other and want to be together don't need a government piece of paper to tell them that.
If by chance worst comes to worst and you guys can't sort it out, then I don't want you to blame yourself, because in that case all it would mean is you learned sooner rather than later that you weren't really meant for each other and then you can move on to find someone who is better for you.
That's about all I can tell you.
