She wants to put every Tom, Dick, and second cousin—plus, her entire office staff—on the guest list. Your wedding is probably the last chance your FMIL will have to show off her baby boy to a crowd—plus, who doesn't want to be surrounded by friends at a big party? Nip the insane guest list in the bud by giving her a guest-list cap and laying out a few rules up front. ("You can invite 30 people to the wedding, max, and only people who have met both of us can be on your list.") Avoid, at all costs, asking your groom's parents (or your own) for their exhaustive potential guest list early on. (Once they've thought about the 100 people they each COULD invite, it will be that much harder to cut the fantasy list.) Figure out the numbers that work for you, and then ask everyone to work within those parameters.
She thinks that chipping in for the wedding gives her the right to call the shots. When parents are kicking in to your wedding fund, sometimes the gift comes with strings attached. To help rein in their expectations, show them your budget breakdown early on. ("We have $30,000, total, so $1,500 will go for flowers, $2,400 will go the the DJ," and so on.) If your FMIL sees you only have $X for something, it will be harder for her to argue for a more-expensive option. If it's a matter of taste—she hates black for the bridesmaids and keep saying they should wear yellow—revert to smiling and your wedding catchprase: "This is what [GROOM] and I have decided to do. We'd love your support." And of course if you don't want her strings-attached contribution (and the opinions that come with it) you can always opt to pay for the wedding yourselves.
She's just generally panicking about everything. She hates the invitations. She hates her dress. She doesn't understand why anyone would get married in December, ever. If EVERYTHING wedding-related sends her into a tizzy, there's probably something bigger-picture going on. Like, she's coming to terms with "losing" her baby to another women—as silly as that sounds—or she's starting to feel supremely old. (I mean, I feel ancient with a son on the verge of kindergarten. I'll probably attend his wedding cryogenically frozen.) If every conversation about the wedding sets her off on a tailspin, talk to your future husband about spending some one-on-one time with his mom. Maybe he could take her to dinner and let her vent for an hour or two. If the drama persists, consider giving her a wedding project to put her energy into: Ask her to assemble the bags for the out-of-town guests or determine the best restaurant for the morning-after brunch. Distraction is a powerful tool—and if she feels like she "owns" a little part of your wedding, she's less likely to bug you about the rest of it.
How's your groom's mother behaving as your plan your wedding?
I'm going to keep my mouth shut for once, since my MIL has been an absolute saint in the babysitting department during Bridal Market.