I appreciate your openness and willingness to talk about this issue. It is something I think others struggle with as well.
Your relationship has a uniqueness to it because there is a declared, strong love, and a separation imposed. This creates a unique dynamic that involves longing and insecurity. In other words, the condition highlights your desire to be together, and the fact that this isn’t possible until December 17th. Naturally there would be an underlying frustration.
Your insecurities sound very natural under the circumstances because so much is out of your control. Your description of your boyfriend is that he sounds like a man of integrity and has been honest with you. Much of what you are describing is very normal for a 21 year old young man in the Air Force reserve. I think you are doing the right thing by explaining your feelings, and for him to explain his. It may be hard to find a resolution that both of your find acceptable until he returns.
What we know about how love grows within couples (see this article) is that there is a mutual vulnerability that evolves between them. Each then learns to be sensitive and care for the other. When your boyfriend returns I would schedule a few sessions with a couples counselor to help with his reentry. I think this would help sort through these very natural issues. You can locate a referral from the find help tab on the top of this page.
Wishing you patience and peace,