I've been reading plenty of letter fics recently so here's my attempt at writing one.
Dear Lisa,
I hope you come back. People come back to life sometimes, why shouldn't you? If I knew how to I'd bring you back myself. I miss you much. The world seems darker without you and everything reminds me of you. I love you and I always will.
But I don't know why I'm writing you this letter, you can't read it now and it won't bring you back but here I am writing it like an idiot. If it would bring you back I'd write you a million letters, maybe I'll write you a million letters anyway. I'm so sorry that I let you down. I'm sorry that I couldn't save you. I hope we can be together again one day.
Forever yours, Ianto