This is not meant to sound negative, just an idea, since you ask!
My own feeling is that trying to fit into something for a special event is asking for heartache and at odds with the slow and steady progression toward a fit lifestyle and the struggle against obesity (not saying you are obese, just a generalization). So maybe a healthier and more realistic way to do this is to tell your boyfriend how much the dress means to you and how it has inspired you to work steadily toward a healthier weight. Hang it in the living room to inspire you and let him see how much you care about becoming the healthiest you can be and how he has inspired you to that. Then choose another dress for the special event and keep looking at your new dress and smiling and say, "Next time!!"
Again, just an opinion! Good luck, whatever you decide.