Dainta,
My husband was also married before. I used to feel that way...threatened. For me it was thinking that one day my husband will wake up and realize I am not who he thinks I am, I am not good enough, that his x was probably better or they enjoyed more stuff together.
Also, he has a past with her. Wouldn't it be SO nice if that part of his life just disappeared? What if her memory and their experiences, in essence their whole life together just disappeared?(

)
There is also his family and friends that may compare the "old" model and the "new" model.
I used to do this. Thing is I had to WAKE UP and SEE that he was not with her, but ME! Something between those 2 did not work out and brought them to divorce. I also had to accept that YES he did have a past life, that not only included an X wife, but his daughters mother (which is yet another different woman) as well. He was with them BACK THEN! Things happened between him and his previous relationship with his daughters mother and probably other women that did not work out. I too have a past, but that IS THE PAST! We are together now, why am I going to let something in his past that I know did not work out bother me? They are not a threat.
For me this was because I was not confident in myself. I did not feel self worth. I did not feel I deserved such a wonderful man, he was too good for me. Now I see that he is great for me, I am great for him, we compliment one another and work well together. WE NEED one another! We are a team. I know he never got that from his former relationships. He used to tell me stuff like that and I did not believe him. (ie:lack of self esteem) Now though I do. I see we work so well together. I see what I do for him and what he does for me. They way we interact, the way we speak to one another, treat one another...I see it and accept it. I no longer deny that I am a good person to him and deserve whatever and however he loves me in return. I grew up in an alcoholic home, dysfunctional so I never knew this, did not know hoe to accept, react, return it. I did not know how to think about it either. My father always had something, some underlying alter motive or reason for everything, there were strings attached...with my husband NOTHING has strings. We are to one another what we are. I was making things more complicated and uncomfortable for not only me, but for US as a couple.
Let go his x. She is done, gone. You are there now. Enjoy what you have now with your husband. You cannot change his past, but you both can create a beautiful today and future with one another.