In a girl's perspective, your girlfriend seems to be one of those insecure girls that compare themselves to everything. In relationship wise, if she is comparing herself to your girlfriend you need to let her know, her and your ex are two different people like their personality.
You need to tell or try to persuade her that you only want to be with her. Let her know how much you love her and make her listen to your every words. Don't let your girlfriend put words for you because that just going to make things rather worse.
Express how you feel about this situation and how its affecting you guys. This will keep her thinking like "Why am I comparing myself to her?" "Why am I being insecure?" Also, like you said, express why would you repeat the things you did with another, with her? If you did, things wouldn't work out like your last relationship. Let her know that you don't want to fall in between those cracks again and wants some changes for the good.
Hopefully this will help you in some way!