And if he is your fiance ... you are engaged to be married ... and should have been dating long enough that you know you can trust him. If you don't know that - you shouldn't be engaged to be married.
Sounds to me like it's less about how trustworthy HE is ... and more about YOUR lack of self-respect and self-confidence. You may want to get counseling to help you with that. Otherwise, it will always be a problem - and probably ruin any relationship you have (including marriage).
realize that he wouldn't be who he is today if he didn't have experiences in the past to shape him that way. that includes past relationships. it isnt realistic to expect a person to have never had a relationship before you. people need to ease their ego and stop putting such destructive ownership on other human beings
RelationSHIP? As in...singular? Well, if he's only been in one serious relationship before you, then he obviously is a commitment kinda guy. If he has said or done nothing to lead you to believe he still has feelings for her, then you really need to see someone about your insecurity.
Most everyone here has a past. Some are good and some are bad. Some didn't work out the way we would have liked through no fault of our own and yet we reserve the right to strive ahead for the pursuit of happiness. All deserve to be happy. Be smart enough to not take the happiness and future happiness away from your loved one, OR yourself. The past is in the past and should not be repeated. Love, Laugh, Live, and continue to evolve.
Understand that his previous experiences and even relationships have molded this man into the man you have fallen in love with. Experiences, learning from past mistakes all make us better people .. so in reality, you should be thankful for his past relationships .. without them, he may not have turned out to be the man he is now.