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I think a letter everyday would be a little too much especially because your not in a relationship with him. You don't want him to get annoyed with the constant letters and having to read them everyday. What I would do would be to write a bunch of letters before he leaves and give them all to him the day he takes off. Letters like "open when its raining" or "open when you need someone to talk to"... that way you arnt constantly contacting him and giving him an obsessive vibe but instead your keeping yourself in the back of his mind every now and then when he opens the next one. He also keeps in mind that the letters were written before he left and he could get a sense of regret when he reads them and think he may have made the wrong choice and my be unable to get you back in the future... which is good. Make him worry a little bit.