Need advice on how to help my girlfriend with insecurities

Hello guys, I'm new here and I hopped on here because I was made aware of a problem that is much bigger than I thought it was. So where do I start...

Firstly, my girlfriend has had a lot of childhood "trauma". She is a second child. And her sister was always in the spotlight. So she had a need to be better than her at anything she could. That problem sticks with her until this day (To clear something me and my girlfriend are both 18). So that means that any opinion matters a lot more than it would to any person without those problems. I will come back to this later.

Secondly, she is very insecure about the way she looks. Even after all of my support, compliments, etc. she will think she is ugly. To be fair she herself acknowledged that those things I say really do mean something to her but it still wont get into her head. This problem also came from her childhood where she was bullied because of her looks.

So now I knew that she was jealous of my ex and we talked about it and everything (that was my first girlfriend 5 years ago, and my last before my current girlfriend). But recently she brought it back up and I figured out it was still bothering her. And not by a bit. She thinks my ex is better because of the school she goes to, because she plays a sport, because (in my girlfriend's mind) she looks a lot better than her. And still after all of my reassurances its not enough. She even said something along the lines of: "if they would to put us both into the same room, all of the people would choose my ex over her in terms of attractiveness". And when I hear something like that I just get a feeling that my opinion doesn't mean anything and that she wants reassurance from other boys or something.

Yea that was my (not so)little rant. Any advice would be helpful. I am also open to any questions.