Tell, her that you want to do Istekhara before the decision. Also request her if she could do an istekhara for herself.
Das Reich brother, I don’t know if anyone here would agree, but I personally think and have learned from practical experience that the beauty of a person enhances many folds after marriage, if the person is good in character. I myself might not have called my wife the most beautiful person alive (before marriage), but after marriage, YES she is the most beautiful person I have seen. I am not trying to falsely console myself. I swear by Allah (s.w.t) I find her the most attractive girl because of the "barakah"Allah (s.w.t) blessed our relationship with. When I look at her, my heart is so satisfied that I never want to look at any other girl or consider anyone for a potential life partner.
I am not saying that you should go ahead and marry even if you don’t find her attractive, but I am emphasizing that at times we fancy someone to be perfect of any flaws (physically) but when we suddenly see the person, the initial shock we get is because the person does not look the way our mind had imagined (free from any flaws). Please don’t rush up. Give your self sometime. Character is more important than appearance, and as I understand from your first post, you might not be able to find many girls like her whose personality and way of thinking is very much what you want in a wife. Usually it is impossible to find a person who has no physical flaw at all and at the same time has the best character and personality.
We need to remember that the prophet (saw) emphasized the most on “character” while looking for a wife. I have a cousin who rejected so many opportunities because of minor physical appearance defects (which is not in the control of that person, but was made naturally like that) but later regretted his decision when he became a bit old, and there were very few potential rishtas left for him. Then he had to chose among the left overs which had less of those qualities then the ones he rejected earlier in life.
If after all efforts you ever has to tell her that you don’t want to marry her, tell her** " You are mashaAllah a very good person by character, and I feel you deserve a better person than me, which Allah (s.w.t) would have written for you. Even after performing istekhara I am some how not convinced, and there might be something better for both of us in not continuing with this relationship"**