My second question, "what is missing in your relationship in that she cheats on you?"
you have some major problems and breakdowns in your relationship if she has cheated on you twice. There is a saying, "fool me once, shame on you. Fool me twice, shame on me."
you no longer can overlook this problem. You need to find the root of the problem. Her cheating is the bad fruit of the root cause.
Since she is your fiance, you both need some premarital counseling before you dive into a marriage which may only have less than 50% chance of lasting. The premarital counseling will either bring you closer together, or have you both agree that this relationship is not what you both want.
You both need to look at your expectations of marriage. You think this problem is difficult to deal with now. After you are married it will be 70 times 70 more difficult because of the legal ties as well if you have children.
Before my wife and I got married, our church required us to get 4 months (meeting twice a month) of premarital counseling with a trained couple. It was the best thing we could have ever did for our relationship. We read and reviewed 3 books "called together", "love life", and "intended for pleasure". We generally completed the "called together" seperately and 3 days before we met, doug and anne reviewed our answers and had us discuss our thoughts, expectations, perceptions, needs. It made us look at issues we never thought about before we were married. We found our first year of marriage was a smooth transition compared to other couples we know who never had this. We have been married over 5 years and our relationship is solid.
If you really committed to her and still want to make the relationship work, get some premarital counseling!