W.J.
L.-
I am coming from the position of someone whose parents were divorced. My suggestion is to first ask your ex to not introduce her to every girlfriend. Almost any psychologist would recommend this...it is normal for your daughter to become attached and then to not understand why the person is gone. It is probably not the most helpful thing for her either in developing trusting relationships as she gets older.
With regards to reassuring her about you and your new husband, that will be difficult seeing as she has already seen you and your ex's relationship dissolve and now is seeing multiple relationships with your ex dissolve. I would be honest with her and talk to her a lot. Explain how much you and your ex love her and that while things didn't work out your love for her will never change. I don't know what the situation is with your ex but I would try to get along as much as possible. My parents still talk and get along which is really helpful to me and my sisters who love them both and certainly did not choose the divorce. If things continue to be a problem, I would recommend maybe talking to a child psychologist...they could probably give you more specific ideas for her age and development. I would also talk to someone before you have more kids with your new husband...this will bring up a whole new set of issues for your daughter who will see her half sibling with parents together as she is going back and forth.
I hope my post is not too forward. Divorce is a really hard thing on kids which I do not think people really talk about. My parents divorced while I was a teenager and it has affected me and my sisters then and now. Just remember to be sensitive to her and validate what she is going through and will continue to go through. Obviously, she will need to move on and accept this as part of her life but validating the difficult parts of the situation will go a long way.
Best of luck,
W.