Talk to him , if you are really worried about your weight you need to know why he has cooled off, it may not be you at all did you ever think of that?, and it might be something bothering him , that is why you need to talk, communicationis the key, all the best .
Dating for just a year? And you're already feeling uncomfortable, wondering where it went wrong and desperately trying to lose weight? If someone doesn't love you exactly the way you are and find you attractive warts and all, spare-tyre and all, then they're not worth wasting your time on.
I love the usual advice, "dump him", "just move on", "if he doesn't love you as you are" etc etc and all that. However in order for someone to love you/fancy you, you must love and fancy yourself. In other words be comfortable in your own skin. Your partner should always support you (the decent ones anyway, sometimes telling you somehome truths), however in order to be loved you must love yourself (in a non ego central way), and find happiness in your own skin. And if you then feel that someone doesn't give you the love you truely deserve, then dump them, and find the love you are truely worth.
You know what the answers are, as you ask for the answers in your own questions. You know in yourself if you feel fat, unsexy, unconfident, that you must work on these first.
My advice. If you were as ship sailing on the open ocean, you wouldn't drill holes in your own boat. So why drill holes in your personal boat.
Learn to love thy self, and work on your own insecurities. Eventually men, women, people will be attracted to that inner self, sense of importance, that you matter.
Birds, I think your case is like the OP. You were drifting apart, and the (imaginary) weight thing was just something for him to hang it on. If it wasn't that it would have been something else. You did the right thing to split.
This is why I think every couple should live together for at least a couple of years before getting married or having children, so that you can have a relatively painless break if you find you are not really suited when the honeymoon period is over.