When a woman looks in the mirror, she doesn't see everything you see, she sees everything she's not.
Thanks to society, the first and "most important” quality a woman is supposed to attain is beauty. That's why women are so insecure. We're conditioned to safeguard this thing we cannot control and quality that we know will leave us one day. We're constantly paranoid and always comparing ourselves to others.
Throughout life, we hear "She's not smart, she's beautiful and smart." "She's not hilarious, she's decent looking with a personality that makes up for it.” The first adjective before any others is about her beauty. All her other qualities come second. When your entire life is based on a single adjective, you begin to think that’s all you’re defined by. We also hear "You're pretty, but not as pretty as she is." "You're hot, but she's hotter." "You're beautiful, but so is she." We've made it a competition. We have made one type of pretty better than another, when in reality pretty is just pretty.
Furthermore, when dating, past relationships can play a huge role in a girl's insecurities. If she has been hurt several times in the past it's gonna be hard for her to believe that you're not gonna do the same, regardless of how much you show her. Also, if she has been blamed for the ending of relationships before she is going to constantly worry that she is going to do the same with you. If her past guys have left her for other girls then she is going to be concerned when you're around other girls because she has been left for other girls "prettier, funnier, or smarter" than she is. These relationships leave scars.
We are constantly afraid that we're not good enough.
Sometimes our thoughts are backed by so much insecurity that they create lies we believe. But, the best thing you can do for your partner is to be there for them while they heal. They need your love and support and for you to help reveal to them how beautiful and incomparable they really are. It is going to be hard and they are not going to make it easy, but if you really love this person, you won't see it as a burden but more of helping the one you love. They need you and they just want to feel loved, I promise.