Sex can be frustrating

So, my boyfriend and I waited a little while to have sex. We definitely did other things. I'm so attracted to him, but lately I feel like he isn't attracted to me. He says that he is, but every time we try to have sex, he goes soft. He stays hard when there's foreplay, but he's only been able to stay hard twice. I'm so sexually frustrated! Plus, I'm beginning to feel self conscious, especially since the pill has made me gain about 15 pounds. I just don't know what to do. It's been 4 months since we've been trying. I just don't know what to do. Any advice?