One thing I will tell you... IF and I say IF you meet someone else; do not break up with him via snail mail. Allow him to assume you are still his love. He needs your support. He will need to know that someone somewhere still loves him. He will become depressed. Do NOT leave him in this condition. Keep your letters flowing on a regular basis.
Send him gifts on a regular basis. Something he can look forward to. Bake some cookies. Send him non perishable items. Don't go to any expense as he may have to share these with his mates.
He's going for his basic training now. But he will leave for over seas soon after.
NEVER show how this separation depresses you. Because his depression will worsen.
Screw his brains out until he leaves and maybe you'll be glad for the break.
He's leaving FOR 3 weeks or IN 3 weeks? If if it's the latter I would give him a big kiss wish him luck and make sure he promises to stay in touch. If he's only going to be gone for 3 weeks...the time will fly and he'll be back before you know it!
The members of the military are such an under appreciated lot, without them how would the rest of us be. That said to be young like you and to begin your life with a long distant relationship is very hard and without intent people do things or things just happen. How to deal with the problem is he is just a boyfriend and you are just a girlfriend, No marriage certificate or promise, is it fair no, but it is the path both of you are on at this point of your young lives. Good luck