Hi, I'd like to help you with your question.
It sounds like your fiancee does like you, but she has an issue with other men.
Usually when someone has trouble being faithful, there is an issue in their background that causes their infidelity. Often, there is some type of child abuse or neglect. Not getting important needs met causes some people to seek to fulfill the emptiness in relationships, especially inappropriate ones.
There could also be a personality disorder involved. Although it is hard to say for sure if your fiancee has one without talking to your fiancee face to face, people can develop manipulative and self centered behavior. This is usually the result of neglect or other unmet needs as a child.
If your relationship is to overcome this problem, your fiancee will need to acknowledge her behavior is wrong and be willing to work on it with you. If she is, then your chances of overcoming this is good. The main issue is trust. She needs to prove to you that she can be trusted again. Once you can reestablish trust, the basis for a good relationship is there.
It is important that you both see a therapist, together or apart, to work on this issue. It is preferable that you see someone together, but if she is not willing to go, then go without her. You need the support right now and someone to help you decide how to handle this.
To find a therapist, talk with your doctor for a referral. Or you can speak to your pastor, if you attend church. Pastors are often very good counselors and can help in situations like this. Or you can search on line at http://therapists.psychologytoday.com/rms/.
You can also work on this together at home. Here are some good resources to get you started:
Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity by Shirley P. Glass and Jean Coppock Staeheli
Infidelity: A Survival Guide by Don-David Lusterman
Private Lies: Infidelity and the Betrayal of Intimacy by Frank Pittman
You can find these books on Amazon.com or your local library may have them for you.
Let me know if I can help in any other way,