My dear --
I stopped reading partway through to share this thought:
Having conversations with this woman, whether mutually agreeable or not, will not change anything. Listening to or paying any attention to any of her faulty opinions or attitudes will not serve you. It sounds like you're already well aware that her ideas and beliefs are not in any way connected to your own reality. So don't attempt to persuade her otherwise. It's a waste of your time and energy.
Now I'll resume reading.
OK. I finished reading your statement.
Have not yet read any of the answers.
I'm guessing you will have received, and will receive, some excellent suggestions and recommendations.
Meanwhile, I'll just focus for the moment on your LAST paragraph.
You are HAPPIER when you haven't spoken to her for a while.
And you're SAD that she is going to miss out on important special things.
My recommendation: For the time being, especially as you plan your wedding, focus on the HAPPY choice. Don't open yourself to frustration and anger and reacting to her crazy-making.
Do not decide for now and forever more that she will miss out on important and special things. It MAY happen, in the future, that she will, one way or another, adjust at least some to (what we think of as) reality, that she may benefit from some therapy. It's not necessary for you to regret now what may or may not happen in the future.
Try taking it one day at a time.
Stay with non-crazy-making people . . .
your fiance, his family, your siblings, et al.
Do NOT spend any time or energy hoping/wishing that your mother will adjust her beliefs and behavior to make it easier for you to have a relationship with her.