You are in a bind. First of all, you need to be careful not to make a common mistake the many people do. Comparing a new relationship with an established one is like comparing apples and oranges. The tame feelings that go along with an established connection can never compete with the passionate and excited feelings that arise in new relationships. So, you don't want to assume that the relationship with your fiance is over simply because the bells and whistles don't ring like they used to.
You might want to read over Dr. Love's Ten Tips: Is it Love or Lust. You will see, when you read those tips, that time is a very important factor in helping you determine whether you are truly in love with this new man, nor simply in lust. It usually takes 6 months to be sure.
You also need to take my Compatibility Test in order to help you determine which of these two men is actually most compatible with you. As you probably know, there are two versions of my test. Version one, you take by yourself, and version two, you take with your partner.
In your case, you need to take version one, two times. The first time, test your current relationship, the second time, test the relationship with this new man. My test will give you a very reliable Compatability Score for both relationships. After you receive your Scores, you will have the information that you need to make an educated decision and choice.
It is brave of you to face the music now, and, if necessary, avoid a marriage that might be wrong for you.
So, take my test and make sure that you aren't comparing apples and oranges.
Lots of luck.