I am not sure what type of diet you are following, but I find that at weddings and other family smachot that are at halls as opposed to people's homes, it is easier to leave food behind. I will have the soup and salad, and the meat but not the potatoes or rice or pasta, and I pack the desserts to take home for kids not attending (if packable) or just dont eat, also load up on pickles and crudites but skip dips and dressings.
I question why you dont like telling your family to take your plate away, or otherwise help you. I find that is very helpful when somebody asks me am I sure I want to eat that. At homes, and with close family, if you are public about your dieting people are generally supportive. That way they can counsel you about whether you should have that extra bite and is it really worth it. If you are not doing everything that you can to get the help you need to stop yourself from overeating, than you are probably not being honest with yourself about your weightloss goals. Do you really think that piece of cake or tasty meat is so worth while that it should set you back a week? I am pretty sure that once you get to your ideal weight there will be more weddings and family smachot to go to and you will be able to enjoy it more knowing you made it.