Welcome to SR.....I'm glad you found this forum but as always, I am so sorry for the reason that brings you here.
Only you can decide what to do. We are very big about self care on this forum. Living with and loving an addict can be draining....emotionally, financially, and spiritually. Right now I would encourage you to do the right thing for YOU. Don't worry about what others might think or how parents might react or anything else.....this is YOUR future.....not theirs. Do what is right for you.
If I was in your spot right now, I suspect I would be thinking.....but the invitations have gone out.....I can't cancel the wedding now. But there are a lot of good reasons to get married and there are a lot of bad reasons to get married. Proceeding because invitations have gone out......when your internal alarm is blasting.......is not a good to get married.
There is absolutely nothing wrong with sending out something that states that the wedding has been postponed. This would give you time to work through the issues before you become further legally entangled with your fiance.
Addiction is a progressive disease. It does not get "better". With a great deal of hard work it can be "suspended" or gotten "in remission" so to speak but it never goes away. And it usually gets worse before it gets better. I don't want to scare you. But love and the promise of life together does not cure addiction. If it could, not one of us would be here.
I'm so sorry that you are having to deal with this so close to a date that is suppose to be one of the happiest of our lives. You and your fiance will be in my prayers. Keep us posted. Ask questions. Read. Educate yourself on addiction. And find support to help you deal with this (Naranon or Alanon are great resources for support). Let us know how you are doing.
We care. You are not alone.
gentle hugs
ke