If you can get sober now and put a solid plan in place I would think that by May you should be able to handle being in public places where drinking is going on. You are of course very early in sobriety and things seem very daunting right now.
You are also still under the false assumption that "everyone will be drinking". Just because alcohol is available at an event doesn't mean everyone will be drinking, many will not. Alcoholics just naturally gravitate towards others that are drinking too.
If it's really still a problem for you at that time, you could just go for the wedding ceremony itself and not attend the reception, or if you do just leave a little early. There is plenty to do in Cancun ( or anywhere for that matter ) that doesn't involve drinking.