You are asking important, important questions. You are also taking a huge amount on yourself. You say you know that you can’t expect a person to change themselves for a relationship but you don’t expect your boyfriend to give you the same courtesy.
It looks to me like you’re doing everything and more than can be expected given your diagnosis. I wish you success in your efforts to be healthy and strong. But healthy and strong doesn’t always mean being model-thin or even typical. From what I read, keeping your weight down is likely to be a challenge throughout your life. Do you want to always live with a “diet policeman”? Does he always want to be the cop on that beat?
Accepting your diagnosis means accepting all of it. For you, that includes doing what you can to work with your doctors and to continue to eat right and exercise to avoid the possibility of diabetes, heart disease, or high blood pressure. For your boyfriend, it means accepting that even when you are doing the very best you can, you are still going to have the medical problem and the issues that come with it. He needs to learn the difference between being encouraging and being demanding. If he isn’t attracted to bigger girls, you’re not the girl for him. Instead of making you feel bad about this, he should be looking at why he’s coming up with that issue at this stage in your relationship. Instead of feeling like there is something wrong with you, you need to be asking yourself why you would love a guy who is this unloving.
You can’t “fix this” on your own. I suggest you take a big step back from the relationship. Work on your own health and well-being. As you feel healthier, chances are you will also have more energy and more interest in a social life. You deserve someone who is going to love and cherish you – all of you. If he can’t come to terms with that, then your current boyfriend isn’t the guy for you.
I wish you well.