You should develop the relationship more and try to find a deeper connection. Vanity can lead to a really shallow connection with someone.

There are two things I can take away from this. The first is that the ex gf is just as attractive as you or that she isn't and looks don't matter as much as finding a good healthy relationship.

Ask him how he feels about you and don't try to live up to some fantasy you are projecting about his past. There is probably a good reason they broke up. She may have been sexy, but with that can come an over inflated ego, and possible a superiority complex. It would suck to date someone like that, who always throws how good they are in your face.

Develop a sense of yourself and some happiness that comes from within. I can say one thing. You will never be the most attractive person because everyone has a different opinion on what that is. Some people like tall and then, other like short and thick. The important thing is what he thinks of you and that you feel good about who you are and can be happy with yourself. Being sexy comes with knowing a person and what they like. It gets developed over time and comes from a meaningful place.

Being objectified is different than being sexy. Throwing yourself at him, and trying to live up to an expectation you have made on your own is surely going to lead you to feel bad about yourself. Try to develop a deeper relationship than one just based on looks. Try to get to know what he likes, what his passions are. Share with him what yours are. Learn to be romantic, go out on dates, have long talks, and spend some alone time together just enjoying eachothers company.

If you just make the relationship about sex, and put yourself out there to be objectified, you are likely to get used. Feel confident in yourself and good about who you are. You don't need to be sexier than his ex gf to feel good about yourself. Remember that she is an ex and apparently being sexy didn't work for her, so why would it work for you. Find out what he wants from the relationship and figure what you want and head in that direction.