I think I would have an honest heart to heart talk with him. I'm sure that deep down he knows that if you continue to lose weight and get healthier, that might have implications on your relationship. I would just be up front about it, something like, "Look, we have gotten into a pattern with this being overweight, stuffing our faces, lazing around on the couch, whatever, and I know I've been your eating partner all this time. I also know that I'm the one changing the game here, and that I'm basically asking you to change your whole life for me. But the bottom line is, I have discovered and come to the decision that I want a new life, and I'm going to get there, whether you come along for the ride or not. I love you and you are my partner, but things are going to change for me. I'd love for you to join me in this, but if not, there most likely is going to come a day when I just can't live this way with you anymore, whether I want that to happen or not."
Sorry to go all Dr. Phil with the dialog

, but why not just say what you've probably been feeling and really let him know what the bottom line is? If, of course, that IS the bottom line. I'm just assuming that because you said you feel like you've wasted the past 18 years, and I imagine that once you start to feel really good, you won't want to waste the next 18.
Good luck!