38G here... and they've gone down thank god since I stopped nursing. But I had double d's when I was in middle school. I've always since I can remember been absurdly big. So I've dealt with these issues for nearly 20 years already!
I've found the belt problem is usually solved if you are wearing a well made correct sized bra. Actually go to a lingerie store and let someone measure you properly. We can switch bra sizes every few months even! They always say 75% of women are found to be wearing the incorrect size. If they aren't completely divided, in the cups entirely (nothing peeking out either side), back strap shouldn't ride up, straps shouldn't be digging in, I could go on and on....
But if they aren't clearly divided and you have no large indentation in the center of your bra that creates two separate peaks than your belt isn't going to stay between them. More than likely that is your problem. I can always tell when I need a new bra because that belt stops sitting where it ought to set. It goes to my neck just like yours but if I have a good bra (and I have to order mine online) it will sit right between your breasts.
Good luck with the surgery if you decide to go through with it. I've kicked it around in my brain for ages. I qualify because of back problems and posture problems for insurance to help pay for it but I don't think I'm going to bother. Breast cancer is high in my family and I figure I'll have to get surgery when I'm in my 40's anyways. :(