Nowadays, many young people have pre-marital sexual experience. Thus, it is in no way uncommon that your fiancee has sexual experience. But now, she has chosen you, and if you handle your relation well, she can become yours for your whole life.
I have had many patients, who are men with a compulsive interest in the previous sexual experiences of their partners. They have been so pre-occupied by their partner's previous experiences, and they have asked so many questions about details about this, that the woman got fed up and left the man who has been so possessed with this.
A sound relation between two partners must be based on trust and acceptance, not on one partner trying to control the other partner through a kind of retroactive jealousy.
The problem is thus a problem for you, not for your partner. If you often have problems with being pre-occupied with compulsive behavior, you might be aided by SSRI medicine combined with cognitive psychotherapy.