How do you deal with insecurities?

I suppose it can depend on how he split up with her, if she dumped him because he cheated on her then your insecurity is a lot more understandable. If not and you and your boy friend have been relatively happy then I don't think you have anything to worry about, he obviously loves you for who you are and not her. It's natural to compare yourself to her but it usually works in reverse with the ex comparing herself to you because it helps her understand what he likes about you that she doesn't have. I suppose its not completely strange because I really don't know your background, some people its just a difference of self confidence where as other people have been hurt before so they are more cautious then others.