The best way to handle this situation is to deal with it decisively once and for all. If you don't, you are playing with a potentially explosive situation in which you might end up losing both ways.You need to really consider the facts and weigh the consequences, and see where it will affect you negatively. You need to make your decision about who you want to be with and though it may be a very difficult decision it's one that has to be made because you don't want to get married to your fiancee' only to find out you made a huge mistake and then you are left wondering what if or even losing one marriage to have another, that is why you need to postpone the wedding indefinitely and you need to do some soul searching. This is easier said then done but in the long run it is you who has to live with the choice you make so it's better to take your time and figure out what you want to do then to just go with the easier choice.
Your fiancee' may not be sure if she wants to marry because she senses that something is going on with you and that your feelings may not be the same as they once were and now she isn't sure if this will be forever. Call it women's intuition or call it cold feet either way she sense that you are not into the marriage as you once were and this may be why she is portraying herself as not wanting this relationship but she realizes that marriage is a huge commitment and wants to make sure that you will be in the marriage both body and mind. If you don't feel you can do that then you really need to be honest with not only yourself but your fiancee' also. You have unresolved feeling for this woman from high school and until you deal with those feeling and either find closure in the past or decide to be with her then she will always come between you and your fiancee' emotionally.
What you need to do is sit down and write both of their names down and then write down their qualities and what about them you love the most and see which outweighs the other you may find that you are just living in the past with the high school sweetheart and that we never forget our first love but does that mean give up everything for her? That is a question only you can answer. You also have to take into consideration your religious beliefs, her parents and would it be worth it to give up your fiancee' for her, would you truly be happy with the other women knowing everything that is working against you. In the end this is your life and the decision you make you have to live with what if you decide to break off your engagement to try with this woman only to find out you loved your fiancee' and made a huge mistake and then it's too late. You may need to stop living in the past and how the relationship was back then and realize that in the present is where you should be, you and the high school sweetheart broke up for a reason and you need to think about what that reason was and think about if anything has changed. Promising to love you till death is a big promise and you have to figure out who would keep that promise and who would let outside influences interfere you don't want to find yourself without either of them and alone in the end.