Any tips on writing to an ex that's in boot camp?

DarkChains
Almost 5 year relationship, was long distance for about a year till I moved into the same state, was planning on moving there since I lived there before and loved it, but meeting him made me move sooner then expected. We had our usual ups and downs mainly cause of my jealousy but I did get over it, but we both loved each other very very much. But he suddenly left on memorial day claiming he didn't love me anymore, I was shocked and hurt, the 2nd time he left but the 1st time was online and a day of silence got him back, this time it was in person and its been over 6 months now.

Since the breakup, I found out he liked my friend who doesn't like him like that, both are going to the air force but she decided a few months before the break up and he decided shortly after the break up but I knew he was considering it and was going to support him if he went through with it. Its like since I found out about him liking my friend, I've grown this hate towards him and became kind of mean towards him, not like REALLY mean but enough for my friend and his to notice something was up.

Saw him last sunday and while he took me home I cried and told him I still loved him, and I don't hate him, just hate what he did to me, he knew all that apparently. He looked like he was going to cry but instead kissed my forehead which he has done on one of our 1st dates, and said 'promise me something, write to me while I'm gone' I planned to but didn't think he wanted me to since I'm an ex that he claims to be completely over but still acts like he is still interested in like he doesn't want to admit it.

Just wondering what I should write to him, have no idea other then like 'hows it going' and stuff like that, I know I probably shouldn't bring up my feelings again unless he mentions it but I'm still unsure what to do.
Any tips on writing to an ex that's in boot camp?
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