Low Self Esteem

I hate the fact that my boyfriend will look at other women. I try to tell myself it's normal because he's a dude. On the other hand it makes me feel like I'm not pretty enough or he finds them prettier than me. I'm afraid he will fantasize about them later and not give me a second thought. It's frustrating for the both of us because I snap at him when I see a beautiful woman and he looks. Advice?