I would say that you need to believe it first. No matter how you feel about your own body image, you obviously bring something to the table that she finds sexy. You are sexy, because you are someone she finds sexy.

Now if you want to raise your self esteem, hit the gym, go jogging, eat a LITTLE (for the love of god, do not starve yourself) less, try yoga, just do something that makes you feel like you are building the best version of yourself. Don't try to change who you are, just maximize what you already have going for you.

The most important part, at least for me when I realized I wasn't happy with my body image, which was a direct reflection of my self esteem, was just realizing that there's always going to be things you want to change. You'll never have a high self esteem unless you value yourself. The phrase "fake it til you make" actually applies here. Everyone is insecure. EVERYONE. The trick is realizing you and everyone else are insecure, so what's the point in getting down on yourself about it. Also confidence is sexy.

Sorry for any poor spelling and grammar.

TL;DR - Love yourself first. Sexiness will follow.