Help please 😞

My boyfriend is in bootcamp for the marines and our recruiter is telling me to write him a letter we haven't been together for a long time. I honestly don't know what to put in the letter. He told me to be strong for him and I'm trying but I really miss him and just to think that next year it's the whole thing over again when I leave makes me really sad. I would love some help to know what you guys think I should put in the letter for him!