Should I worry about my ex boyfriend so much (Help Plz)!!?

We're both 19yrs old, he's a Marine serving in Iraq. He gets back sometime in April and although we're not together now, we plan on getting back together. I just feel like I'm the one always reaching out to him, rather its through emails or letters. I know that he's busy and I don't wanna add stress to his life, that's why I haven't even talked to him about this, but I feel like if he has time to talk to other people, like friends and other girls, then why can't he take the intuitive to talk to me. I would think that if you wanted to be with someone then you would talk to them whenever you have the chance. He's always telling me that we're gonna get married later, but im not sure if we should if he's still talking to other girls. I know of one girl that he's slept with and he wished her a happy valentines day, and didn't tell me anything. I feel like if he wanted to be with me, then he would stop talking to her. Why do I worry about him when he barely thinks about me?