Thank you very much for asking us your question, but you are not likely to be happy with what I have to say.
Sometimes the things we want are not the things that are the best for us. While I very much appreciate your love for this young man there are several features that scream out here. Nothing in this scenario has the makings of a foundation that will sustain a life-long relationship. Very few people are ready to get married when they are 17, and I would encourage you to seriously rethink your situation.
At some point studying relationships is like understanding physics. Certain rules are going to be in effect whether or not we like it. Imagine holding a delicate ceramic cup out the window of a two-story building and aiming for a marshmallow to land on safely and not break. Getting married at 17 and expecting a life-long relationship to evolve happily is equivalent to this feat. It could happen, but chances are against it.
Marriage in general is a risky undertaking and the divorce rate is high; nearly half. If you are serious about making your commitment to each other make the promise and see how it goes. The piece of paper saying you are married can be added down the road if desire is still there. If it feels safer for you to say you are engaged then okay, but I would not have a wedding until you have spent time with your boyfriend after he returns. This is a particularly tender time in your lives, and you are both likely to change radically over the next several years. Your perhaps future fiancé is going off to war, and this will require him to readjust to civilian life when he returns. In other words, you will both be different people when he comes back. Learn about who you are; you can decide what kind of a relationship you want when you have reconnected. That will be the time to see if you want to be together, not now.
Your mother simply does not want you to be hurt. A parent’s instinct is to protect the child. Your mother has a good point. He hurt you and the relationship ended badly, and now he is going away – another hurt I am sure your mom senses coming toward you.
If you love this guy then stay connected to him while he is away and when he returns see if a relationship evolves. If you need to be engaged you should at least wait until you are 18 to marry, but I would recommend against it. Develop yourself while he is away, then see if there is enough to build on when he is back. If you choose otherwise, I wish the marshmallow and the cup good luck.
I would also seek therapy while he is in Afghanistan to cope with the separation as you develop. You can find a referral at the top of this page under “find help.”
Wishing you patience and peace,