Maybe there is something lacking in the relationship. Like, he is not as sweet as he used to be, or he doesn`t give you the attention he used to, and so him looking at other girls is making you feel really insecure, when deep down you know it means nothing. Don`t you notice attractive men? When you do do you think about leaving your boyfriend for him?
Anyways, I think what you should do is start taking care of yourself, to make yourself feel pretty. Find something that will help boost your confidence. Maybe work out, or eat healthy, or buy new make up, or get a make over, a new hair style, buy new clothes - clothes that make you feel sexy... Just do something that will make you feel like a different person, a person you could look at in the mirror and say, damn she`s hot. If it`s not looks, then set a goal and accomplish it. (For me, accomplishing something boosts my self esteem. Because I do get insecure too. WHen it`s really bad, I usually change my hair style, or do my nails, or buy new clothes. It usually helps)
When you start focusing on yourself more, you won`t feel too insecure and it won`t bother you so much when he looks at other girls. Also, remind yourself that he is with you, and he is just looking. I notice attractive men, and women too. It really annoys my friends, and boyfriend (or ex, whatever) because I get distracted a lot. But it doesn't mean I want to date any of them. I just admire their beauty.
I used to get really jealous when my exex would look at other girls, so I talked to him about it, hoping it would fix it. But it didn`t. It just made it worse. I just started feeling insanely insecure, and knowing that he knew made it harder for me.
So try focusing on your body, and flaunt your assets. Because when YOU feel sexy, people around you will see it. And another thing that helps insecurity is when people start checking you out. I know its lame, but it really helps.