Nervous
Sherlock is alternating between escaping to his mind palace and pacing back and forth in the hotel suite, much to the chagrin of his best friend.
"Sherlock seriously, you have got to calm down."
"There is no need for me to calm down." Sherlock hisses, momentarily stopping to glare at John. "I am not in a condition that requires me to calm down. I am perfectly calm."
And then he starts pacing again.
The ex-army doctor groans. He's seen Sherlock Holmes maintain his composure in the most nerve-wracking situations, but when it comes to things he's not supposed to freaking out about, he's more afraid than a toddler experiencing his first thunderstorm. "You are not calm. That is understandable, but really, you have nothing to be nervous about!"
"I'm not nerv—"
"Yes you are." John insists. "It's normal but you of all people know what you mean to Molly. She's not going to leave you hanging today. Has she ever left you hanging, or alone, or lost? She's never humiliated you and you know she's always there for you. What's making you so unsettled?"
Sherlock sighs, slumping down onto the lush hotel couch. "I knew that…" he mutters under his breath. "Just… what if she—" He takes a deep, shaky breath. "You know how terrible I've treated her in the past, John."
John looks at the detective with a gentle smile on his face. It makes him really happy to see his previously romance-allergic best friend now irrevocably in love with the amazing Molly Hooper. "That's in the past. Yes, sometimes you do still hurt her every now and then, but if she didn't leave you when you were constantly being mean to her, when you didn't give a toss about her feelings for you, what makes you think she'll leave you now?"
Sherlock is silent for a bit. He doesn't smile, but John knows that he's comforted.
"Thanks." he says simply, to which John laughs heartily.
"No problem, mate. Now can we concentrate on me? This is my wedding, after all." John says, moving to the full body mirror and checking his appearance for the millionth time. "Geez. The way you're acting, you'd think you were nervous about getting married, not freaking out about asking Molly to dance."
Note: The author can definitely see this happening. Considering his usual detachment from social cues, and the imagined panic at realization that he actually would like to dance with Molly in that not-just-friends light, it'd be a nice thing to see him actually be the one who's all nervous and fidgety when he isn't even the groom.