After seeing a certain tumblr post, I felt this needed to happen. Maybe it did off camera (Up to a certain point)
"You said something interesting." Sherlock said, seemingly out of the blue as Mary entered his hospital room through the nearest door. She looked up from where her hand rested on the knob, surprise barely etched into her features.
"I haven't said anything yet."
"Not now. Back at Leinster Gardens." He stated, giving the older woman a look that clearly said 'keep up'. She crossed her arms, looking down at the detective as a doctor entered through the far door to clean up the conjoining bed.
"And what did I say that was so interesting?" She asked, arching a fine brow at him.
"You said there was nothing you would not do to stop 'that' from happening. To stop John leaving you from happening."
"Well it's true-"
"You can't stop it." He cut her off. Mary's eyes widened and she scoffed, a disbelieving smile on her face.
"What do you mean?"
Sherlock took a deep breath, the light smile on his cupid's bow lips fading as he sat his bed up.
"It's not a threat, mind you. It's a warning." He added before continuing, looking her in the eye. "I have done everything I could to stop 'that' from happening. To stop him leaving. To stop him finding someone else. And I'm telling you now if he chooses to, you won't be in his way no matter how hard you try."
"Then I'll stop him wanting to choose to."
"Ugh, repetitive." He groaned, ignoring the twitch of annoyance in the woman's brow. "You don't think I've tried that. Now that's adorable.
"I leapt off a building for that man. I let people I despised humiliate me and ruin my career and reputation for him. Then, spent two years completely isolated from everything and everyone I cared about, enduring various forms of physical and psychological torture because the very thought that there was a chance of meeting him again kept me going. And when none of that worked, I stepped away, let him be happy with you, helped you even after you shot me in the chest."
"Why then?" She interrupted calmly. "By all rights, you should still be fighting me for him."
"Because you're the one he chose." He returned the interruption, expression slightly defeated. "Because even after everything, John chose you and I would not dare argue with his choice because his happiness is the most important thing to me."
Mary looked down, the pair listening to the doctor on the other side of the curtain exit.
"I died and came back to life for him, and I'd do it again in a heartbeat. So keep that in mind before you say you'd do anything. Because I did everything, and I still couldn't stop it."