I don't think this will be much help but, I think you should stop bringing it up. Being married isn't that big a deal, and since he has already committed himself to you, I wouldn't worry about it. What's the difference between right now and having a ring on your finger? He's still away for long periods of time. I know that you bringing it up is probably irritating to no end, and it's only causing you stress and anxiety, so you need to just think about a few things in order to reconcile this in your mind so that you can be content in not thinking about it or bringing it up:
1. Why is this so important to me?
-Am I afraid that our relationship is not as "serious" or as "committed" as a marriage?
-Do I fear that he will not have enough reason to come back to me or that he will look elsewhere for love?
-Do I need to justify this long distance relationship by having a ring, thus a "solid commitment"?
2. What does this make my boyfriend feel when I bring this up to him so much?
-Does he feel as though that I don't take him as seriously because he hasn't given me a ring?
-Does he feel as though this is eating into what little time he has to see me?
3. Does he already have a plan for proposing/being married?
-Maybe he wants to wait until he can be home for longer periods of time before he feels comfortable committing to a marriage, thus a "stable life".
-Maybe he wants you to stop talking about it so that it doesn't spoil the surprise when he DOES propose.
-Maybe he honestly doesn't have the time right now to shop around for a ring or the money to buy a ring.
I really think that you sitting down and really thinking about this issue by yourself or with trusted friends and family will benefit you. An obsession with being proposed to and getting married, even when the question of "if" has already been taken care of is usually a sign of insecurity. I think those deeper down issues are the more meaningful ones that you should be bringing up and dealing with, whatever those may be.
Good luck to you and your boyfriend! I hope he does well and you two have a really great time together (and that neither or you annoys the other!).