@Seed,
I remember at the very beginning when things were going wrong with you and your wife and you were so hurt. Now that you have climbed over that particular mountain, someone has just plopped another mountain in front of you! Typical!
You have to listen to your heart and your head. If you are no longer interested in making her your life partner then you have to be honest with her, but sweet about it so she does not turn against you or feel insulted. I know it's hard and I know you don't want to have to tell her no in a way that is offensive.
Tell her that you are lucky to have the most wonderful woman as the mother of your child and you hope to always have her as a part of your life. Tell her that you will happily shake the hand of the man who is lucky enough to become her life partner and who will love her as she deserves, and ask her, when the time comes for you to find the woman you are looking for, that she will also be happy for you too.
Tell her how lucky you both are to have met, found young love, shared the joy of a daughter, and now are content to be good friends and co-parents.
Every time she tells you (or texts you) that she wants you or loves you or wants to get back together, respond with something that translates to you two being lucky to come out of a relationship that wasn't right and being able to transform it into good friendship with good parenting/communication. The more often you put it on this basis, the sooner she will realize you are not biting and not interested in getting back together.
I don't know her at all of course so I am not sure if you being completely direct with her and saying "I'm not interested in ever getting back together with you" would be too much. Not sure if she would find that a bit of a slap and resent you and maybe not be as giving with access to your daughter. I would be careful of putting any of that at risk.