My insecurities are ruining my relationship

Recently I started becoming incredibly insecure. I've been with my boyfriend for a year this month but we started having problems due to my extreme insecurities. I started gaining weight shortly after we begun going out due to health reasons that didn't allow me to work out and made me quit cheerleading.

I was always a very active and was never overweight. I'm 5'7 and currently the biggest I've been in my life (size 12) and my boyfriend won't stop bringing up the weight issue even though he can see I've been trying so hard to lose those extra 30lbs. I keep a diary of what I eat using an app on my phone, I ran the London Marathon but my weight keeps fluctuating. I put weight back on the minute I stop working out for lets say a week. I just lose and gain, lose and gain.

I've been so frustrated with this process that my self esteem has vanished. I used to be the girl who would wear tiny shorts and have her tummy out and now I'm the girl who hides behind layers and layers of fabric. My boyfriend keeps asking me why I'm not losing weight... He is discreet and subtle with his suggestions but I can't help it but get more and more insecure about my body. He might say that he still loves me no matter what, but this is not what I hear. Every time we see a good looking girl I just instantly turn depressed. I never go out anymore because I don't drink due to all the calories and I feel ashamed of myself being surrounded by girls looking amazing.

Every time my boyfriend talks about other girls I now get incredibly jealous and whenever he goes out I just get more more insecure. I know he wouldn't cheat on me and I trust him it's more about the fact that I'm not confident any more. We've been arguing a lot lately and all of the arguments are caused by my own insecurities. It's like I cannot help myself. I used to be the girl that would never get jealous or insecure and now a bin has more confidence than I do. All of his friends are good looking and especially the girls. Every time we're on his facebook I see all the girls in bikinis popping up on his dash and it just kills me little by little. It hurts so much I'd rather be single even though I love him to bits. I've heard his flatmates talking once about me getting fat and how he should break up with me. When I'm not with him I get so depressed I think I should just give up on our relationship because of all of my insecurities getting in the way. Help!