Body Issues

So I'm not comfortable with my body, though I'm trying to move away from the negativity. My bf is a very positive force in my life for it, but every once in a while it's almost as if he forgets how hard it is for me to come to terms with how I look. I know I have one body and I should cherish it, there's just some times where looking in the mirror makes me want to cry.
My relationship is long distance, and my bf wants to start being more intimate, firstly when we are FaceTiming. He's the first person who's ever seen me mostly nude. I'm a plus size girl, and my past relationship never got as serious as this one has.
So we were talking and things progressed, though he kept suggesting a little more and a little more- but I didn't know how to tell him "not yet" without making him upset. In the end, he still was upset and before I could apologize, he hung up.
I don't know how to explain how it makes me feel because he'll say, "but you know I think you're gorgeous." I, myself, don't feel gorgeous. I don't feel beautiful. After being told for so long how disgusting I look, it's hard to automatically feel beautiful. I ended up crying after he hung up. I don't know what to do; I feel like I've messed up. I'm so happy in this relationship, but I don't want to ruin it with my insecurities. Any advice?