Loveliest ex of a guy that I’m dating with

I saw the picture of the guy, whom I dating, with his first ex before and damn she’s way better looking like he had the perfect girl ever before. They broke up after being in a 5-year long term relationship in high school coz the girl cheated. Now, I randomly thought that they looked good together like a perfect couple and I feel down to myself coz I’m not pretty like her. How should I handle insecurities. Please help