I have a problem with my writing, whenever I write fan fiction I can get my tone of narration right, I can get it to sound natural in my head and have the right sort of wittiness that I'm looking for.

When I write original fiction? Not so much. No matter what I do the narrative is dry or forced, and I can't seem to get a hold of that voice that helps me write my fan fiction.

The fact that you can write fanfic with a good voice should tell you something. You know the characters, you have confidence that you can speak the way they would. Now you need that same confidence with characters you have created from scratch.

Try this. Start with one of your characters. Pretend another character, someone you are familiar with and who is similar, is telling that character's story. Eventually you will come to places where your character's voice will deviate from the other one's. In other words, your character will raise his/her hand and tell you, "That's not how I would narrate this." That is what you want, a character that comes to life, that knows who s/he is, that is unique.