If you are having doubts it is probably a good sign that this is not for you. I would sit down and have a heart to heart with her...that is the only thing that you owe her. You should not feel bad for your thoughts because they are your own..you cannot change that. However, it is important to be truthful with her because the worse thing that you can do is decide to carry this out longer if you are not 100% sure that you want to marry her.
If you love her or loved her ever then you have to do this.

Jessica