You should spend some time reading stuff on this forum...spend a few hours, days, whatever, reading through people's stories and questions, etc.
That should give you more of a perspective than you're getting from others.
Open marriages and other forms of non-monogamous relationships can be quite successful. Lots of people figure out a non-traditional relationship style that works for them.
But, I will also say that if you are interested in an open relationship and your fiance is not, then the two of you are not ready to marry each other.
Single, straight, female, solo, non-monogamous.