Where doesn't it fit you exactly? Around the bust, waist?

If I were you I would go and find a good corset, a cincher if it is the midsection that is the problem. You will need to find a real corsetier, not like one from bras and things or something, but a proper corset, steel boned, but it will easily take you in enough to wear the dress for the day without the unhealthy aspects f having to lose weight - which you may not lose from the place that is preventing the dress from fitting.

GL

Either way though, you need to tell the bride.
The dress could also be altered to be a lace up at the back, so you can wear it that way, but bride will need to know where you are at.