John and Sherlock are getting married. It was supposed to be Sherlock's happy day, but he's extremely nervous. Little things go wrong (a stain on his shirt, a traffic jam on his way to the wedding as he and John go there separately, and so on). He's very nervous and angsty and worked up. Maybe there's something that makes him (erroneously) deduce that there's a chance John might decide to call the whole thing off in the last moment. And maybe that fact makes Sherlock contemplate running away and never see John and their acquaintances/guests again. It's not like he's made for any of it, right? Maybe there's even some crying on his part.

He just wants the day to end as soon as possible, even if it's going to be a complete fiasco.

Good ending is preferable though.

I'd love it not to be an omegaverse, but if it is, I don't mind at all, as long as it's Omega!Sherlock/Alpha!John.