Insecurity is definitely unattractive, but, to be fair, your girlfriend has good reason to be insecure in your relationship. You have been putting another girl ahead of her in your mind for most of the time you’ve been together, right? And I’m pretty sure she knows that if your ex came back, you’d drop her and go back to the ex. That’s definitely enough to make anyone insecure. It’s good you can see how your insecurity might’ve affected your relationship with your ex though, too, so you can at least take that lesson from it.