Kerry, I am going to preface this by saying that, as with all advice you will see here, it is just someone's opinion that you are free to take or leave, and that others will likely have different opinions. I am also going to say that we are glad to have you here and hope that you will continue to post and share the journey with us. Please feel free to jump into the forums as you please.
So, my thoughts on this.
A lot of the cliches come to mind, first off. Like, if he doesn't love you for who you are, he's not worth having anyway. If your weight is enough to break up the relationship, then how much of a relationship was it, really? And, if you are trying to change for someone else, that's not the right reason. But I hate to just throw those things at you and leave it at that, so here are my other thoughts.
First, I hear you a little panicky at the thought of losing him if you don't change, so I would suggest that you back up a bit, think about it logically, and realize that this isn't going to change overnight. Weight that comes off quickly comes back quickly, so take it slow and steady and you will be healthier and happier. Second, make sure your priorities are in the right place and that you're doing this for you. Whether or not you remain in this relationship, you want to get something out of your efforts...and...here's the important part that a lot of people miss, you are worth it by your own self.
So I am happy to offer a suggestion, but it's not a quick diet fix or magic solution. Sit down with your partner and tell him that you appreciate his honesty. Tell him that you want to make a change, and you are doing it for you [insert reasons you want to do this for yourself] and also that you want to do it the right way and that means not overnight (you didn't say how much you want to lose, so I'm just winging it here). If he loves you and is worth your while (IMO), he'll man up and be supportive and not critical. If not, then you just might deserve better