LuckyCharm - my heart goes out to you because this has happened to me; except it was my wife at the time.
I think what we have to realize is that that person who has hurt us has the same 'right' to disobey or dishonor God as we do. By exercising our rights to sin or not to sin, we impose a consequence on the lives of other people. God must allow us this right because He chose to create us this way. It was not His will that you were hurt by your fiance's actions; but what He is looking for from you is how you choose to respond to being treated as you have been. As hard as it is, you (and I) have to be able to say to God: "OK Father, this person you brought into my life (if we really conferred with Him first) has chosen someone else. I don't know why you wanted this to happen, but it's your problem now. Help me accept what I cannot change and show me your will for my life now."
The relationship has to be turned over to God, just as our possessions, our money, our jobs, our kids must be surrendered to Him. It's all His. We can only use what He has provided us with. Our responsibility is to be a faithful steward of His blessings. Even in something as intangible as a relationship, we must be faithful not only to the person God has brought into our lives, but also faithful to God by working hard and building on to what He has blessed us with. He expects a return on His blessings; in this case a closer relationship where two become stronger and more efficient than one.
In my case(s), I did not consult God before wooing and marrying three women. These were not the mate He had chosen for me, the person that He knew would complete me and I her. As a result of doing it my way instead of waiting on God, I've divorced three women, lost custody of five children, created crushing debt, and have been unhappy for many, many years.
A year ago, God brought the right woman into my life. He blessed me and has given me another chance. I am happy, but there is a problem. She lives over 2800 miles from me. My responsibility is to take what He has provided and build a relationship that will honor God by relying upon Him to make me a Godly husband and father. I'm even having to rely upon Him to bring us together so I can be a blessing to my fiance and our children.
I thought that I was being punished when my marriages failed. What may have happened is that my divorces were blessings in disguise. Sure they have cost me emotionally and financially, but the joy I have knowing that this is the right woman will far outweigh the pain of the past. Perhaps your fiance was not God's choice for you and He allowed this to happen so that the real mate can come into your life.
As far as God changing His mind. I believe that He has done so in the past and He continues to do so. He is not a static, inflexible God and one has nothing to do with His infallibility. God knows who will and who will not accept His offer of salvation, but that doesn't mean that He's withdrawn it from those who won't take Him up on it. We were created in His image and we can change our minds. Look at it this way: when we're born, God has already determined that the sin seed in us keeps us apart from Him. However, when we meet Christ and accept Him as our Lord and Savior, God no longer considers us as lost, but as redeemed. He looks at us as being faultless where before we made a decision for Christ we were stained. He's not pretending we're His children, He's changing His mind about it.