This has probably been done a thousand times over but I wasn't sure and I wanted to give it a try. I know it's short, but I have a couple written. If someone likes it I'll post another :)
And in case it wasn't obvious, I don't own Sherlock.
Sherlock,
If you were here you would say this was pointless. A waste of my time. But I miss you too much to be reasonable. I like to think that if I write you these notes that somehow, someday, you'll come back. I want you to be able to have something in that case. Something to read, so that you can catch up. I miss you Sherlock, and I still want you to be a part of my life. I refuse to grasp the idea that you've left me for good. But for now, I'll just have to deal I suppose. Wait it out until you return. The flat feels so empty without you Sherlock. So big and quiet. I bought a dog to keep me company, and he's helping a little. But he's not you. I named him Gladstone, and I like to take him for walks and sit with him on the couch when I read or watch the telly. He keeps me busy, keeps my mind off of you even if it is just for a little bit. But I wish you were here instead, insulting the people in the shows or the books I read. I wish I could hear your voice for real, instead of the memories that haunt my dreams. I wish I could see you again or hear your horrible violin playing again. But like I said, I'll just have to wait. And go walk Gladstone.
—John