Hi Lore, thank you for the question and I'm sorry you're in that hard situation. Let me start by simply stating that relationships are hard, no two ways about it, especially in the close relationships like parents and siblings and spouses. They bring us some of the best feelings and loveliest moments of life, and they can cause some of the greatest heartbreak with the bitterest of feelings. Probably at least half of the situations I deal with as a mental health provider involve relationship issues as a central problem. So, if it brings you any comfort, which it might not, just know that you're not alone in your struggle.
Well, to begin examining your specific question, I'd want to know more about the reasons why you don't want to attend the wedding. Is it your own unresolved hurt or resentment from your parents divorcing? It is your genuine worry or concern about the relationship not being good for your mom? Or is it some other reason? You have told her you don't support the union "for several reasons" so maybe it is all of the above and more. Did you express those concerns to her early in their relationship or was it just when they announced their intent to marry? How did she respond to you when you expressed your concerns? Sorry for all the questions, that's just how my counselor brain works. You obviously won't be able to answer my inquiries, unless you decide to engage in ongoing counseling with me, but maybe you’ll let those questions help you reflect on the situation and gain some additional insight from them.
So, without enough information to give you very specific answers, I'd like to share some of my general thoughts with you at least. It seems like this problem really comes down to boundaries and how firm you are willing to make them with your mother in this instance. Communication of boundaries can fall on a spectrum from aggressive to passive, with assertive and passive-aggressive falling somewhere in between.
Assertive communication is usually the goal we strive for in relationships we want to maintain, but for a variety of reasons even in those relationships, we sometimes find ourselves being aggressive or passive or both. An example of aggressive communication in this situation might look like, "I hate your fiancé and I hate that you're marrying him, and I never want to see either of you again." That might not go well toward any future relationship with your mother, if that's what you want. A passive example might be, "I'm so selfish for not supporting my mom in this, I just need to bury my thoughts and feelings about it so I can be there for my mom." Passive-aggression might look like, "I'll go to the wedding but I won't like it and I'll act miserable the whole time so my mom and that idiot fiancé of hers can see how much I disapprove." And there might be a lot of different assertive examples we could think of, but they'd all look basically like this; "mom, I understand that you love this man and probably have your own good reasons for wanting to marry him. My concerns are (insert your reasons in simplest most straightforward terms possible), and for those reasons I can't genuinely support you marrying him. I respect and love you, so I want to be honest and clear with you to prevent this from becoming a wedge in our relationship going forward."
Then, you've maturely and authentically put the ball in her court. However, a word of caution here, you get to choose your boundaries and actions, and you can even ask someone else to respond to you with certain thoughts and feelings and actions, but when the rubber hits the road you don't get to choose their reactions. You have to give others the same freedom to choose that you’ve taken for yourself. But, that's no reason to just throw your hands up and ignore or go against what you genuinely want in order to control or diffuse another person's thoughts, feelings, or actions.
If you ponder the questions I asked above about the reasons you don't support the union and don't want to go to the wedding, and if you assertively express thoughts and feelings to your mother with the boundary that you will not attend, and she still cuts you out of her life, then the problems are deeper than you just not supporting this marriage, and I'd recommend a course of family counseling. But I trust that with these ideas you'll be able to get through this rough spot with your mom and come out the other side still loving each other even with your differences and boundaries. Best wishes!