Jealousy ? Am I over reacting

My boyfriend is amazing and truly loves me with all his heart and treats me with the upmost respect. I have had two kids and I'm size 14 but I do have extra skin and stretch marks from my pregnancies so I am overly self conscious of my body! I have very low self esteem sometimes. 
​Ok so sometimes when we are out shopping I notice him staring at other girls and they're all real skinny, perfect bodies and very pretty. I will look at him as he's doing it so he knows I see him. Then he will get kinda embarrassed and smile knowing I saw him. But it really makes me feel so insecure and It causes me to have melt downs and get upset because I'm trying so hard to lose weight and I feel like I am not good enough or he wants better. 
​I have no self esteem and I work out everyday and it helps a little. But I still see all these skinny perfect body ladies that have had kids. Makes me so jealous. 
​So am I over reacting?  How can I feel better about this. I don't get why I'm like this. He adores me and thinks I'm beautiful. And wants to marry me But I'm not beautiful like the skinny girls. It has even caused multiple arguments as well.