If you feel that way, just talk to her, and say that it mught be best to wait awhile, remember, when you get married, it is supposed to be for keeps, and if you feel the way you do now, think what will happen later,also it rally would not be fair to her, or to you to get married until you are 100% sure luck jo
Is it cold feet or something that REALLY bothers you? Do you want to be happy with this women? Past relationships? I mean don't we all have past relationships that help us become who we are unless she cheated and that's your concern I wasn't really clear there. But what I am saying is, was this an issue two months ago? Or when you asked her to marry you? Or is this just a bit of cold feet and you are scared? Or do you need a way out? Think about this.
I don't see why her past relationship affects you so deeply, unless it's a relationship that was in the past but it's also a part of the present.
What I mean is, is the person she had a relationship with before you still involved in her life in some way on a regular basis? That happens sometimes, like if the significant other has a child, so the child's other parent must still be involved, or if the significant other works with the person of that past relationship.
If that's the case, I can see why you have these insecurities. That, or if she did something that emotionally hurt you, like cheating.
But if it's just a past relationship that's over and gone, why does it bother you so much? I think you really need to re-think marrying this woman if you are having such an issue--solve your problems first, then make a committment if the problem can be solved.
Teko said it best, "Until you get a grip on the past, you cannot have a happy future."