It is really hard to deal with this kind of insecurity, but you cannot change what has happened, you can only change the way you deal with it.
You have to remember that our experiences make us who and what we are, and that applies to past sexual experiences as well.
Remind yourself that he is with you now, not anyone from his past, and fretting over what he may or may not have done is only going to hurt you, and your relationship.
You must try to affirm yourself as an individual - make sure you have a good circle of friends with whom you are comfortable, and try and confide in someone who will reassure you.
Do not put all your energies into this relationship - it is unhealthy, and will be destructive.
You are mature attractive rounded interesting individual, you just have to make sure you remember that - he obvious does.