I pointed it out earlier this year that weddings make families do the wacky. But the wacky factor can rise astronomically when your parents are divorced. Your wedding involves the same things their divorce did: your mom, your dad, and you. You know how well that went the last time. And, this time, it's all going to play out while you're planning your wedding, and in front of everyone else on the big day.
Whatever the current conflict is, not only do you have to hear about it (all of it), but you have to mediate, too. Yeah, I've seen this many, many times before, so the good news (such as it is), is that you are not alone. But it's still a complete pain in the butt, and just adds to the stress you're already going through. What do you do? I know it's not going to be easy, but here is what I've seen work for my couples:
1. Treat your parents like the adults that they are. You have no idea how much flack I've gotten for this suggestion. Do not treat them based on how they're behaving, treat them how they should behave. Adults, at least theoretically, are able to see reason. They know that speculation and irrationality are just that. Dad said he'd help with the wedding costs, but your Mom is certain that he's going to be stingy with the money. Your Dad is constantly asking if he's going to walk you down the aisle, afraid that your Mom is going to demand that her new husband do it. Just say, "Mom/Dad, that's not going to happen," and change the subject as soon as possible. Remind them that you're not choosing sides, you're just not worried about whatever they're freaking out about, because, as you just told them, it's not going to happen. And again, don't linger on the subject. But, be prepared to go through this spiel more than a few times. Practicing patience will prepare you very well for marriage. Having them both walk you down the aisle? Yes. Having them sit at the same table? No. Hedge your bets on the side of limited contact.
This is why people hate this advice. It's hard, and you're not used to doing it. I get a lot of "how am I supposed to treat them like adults when they're acting like children?" I'm not sure what your point is, there. How was treating them like children working for you? Try something new. You can always go back to the old way if it fails. It's hard. Do it anyway, it's worth it. This will prepare you for marriage, too.
2. Act like the adult that you are. That means no tantrums: don't scream that they are ruining your wedding. Don't yell at them for not being better/nicer/happier in the meantime. As an adult, you know that you can't change the way your parents are, and have always been. This is pretty much it. You can only change how you act with them. If you do it often enough, it will change how they act with you.
An adult makes a decision and sticks to it. Don't let yourself be bullied into something different based on your parents' emotions or motives at the time. Memorize this and say it to them as needed: "I really don't want this thing you're suggesting, and I really love this thing over here that I've already chosen."
3. Remember that they both have your best interests at heart (hopefully) but that each parent has a different idea of what your best interests are. Mostly it involves the other parent backing off or possibly disappearing altogether. They love you, but they can't see past their own feelings. Remind them that you love them both and are really happy that they're both going to be there to celebrate with you, and that's all that you want. Ask them each how they would like to be involved further and involve them. Thank each of them for their help. People love being appreciated. Appreciate them.
4. Nine times out of ten, whatever they're butting heads about isn't about you, at all. It's about all the reasons their marriage broke up in the first place, the PTSD from their failed relationship. You were there, you know what I'm talking about. It's about the resentment they have about all the unresolved feelings and issues and blahdy blah bling. I'm not being flippant, but the point is this: IT IS NOT ABOUT YOU. When you have to hear, for the 15th time, that your Mom has a problem with your Dad bringing his 25-year old girlfriend to the wedding? That is so not about you, and you know that. Ignore it, or if you're feeling feisty, calmly remind her that you told him he could, but if she really has a problem with it, she can talk to him about it. Go scream into a pillow later. It's not about you. It's not about you. It's not about you. Breathe...
5. Your future marriage is not their past marriage. But I'm going to be honest: If you're afraid that you two might turn out like your parents, you're right, you might. But bad marriages are built; they're not transmitted like an airborne virus. A bad marriage is created by the couple who is in it. You have a pretty good idea of what broke down your parents' marriage, so avoid those things. You know what makes your relationship awesome, so keep doing those things. Again, you can only control your own behavior and your future. You can't do anything about the past except remember it, and learn from it.
Look, I...have parents. I know that they are not always easy to deal with. But, I always think about what Mark Twain said, "Some of the worst things in my life never happened." It never turns out as badly as it plays out in your head. I can almost guarantee that there won't be a fistfight. I am fairly certain that they will be civil to each other. And I know, without a doubt, that you will still have a fantastic wedding day, surrounded by everyone you love. Enjoy it.