A Little Word Of Advice;
Whether he just left, is going too or has been for a while. ALWAYS stay positive! It is the one thing that will help you get through this. I know I don't have too much experience yet but I think I can help a little. My mom waited for my dad while he went on his mission. I can't say they are still happily married but the mission thing wasn't the reason why they aren't together. My mom wouldn't change waiting for my dad, she grew so much stronger from the experience. And I know all you missionary girlfriends out there will too. Hang in there and stay strong for your boy. When writing letters or emails, what you wanna do is like I said STAY POSITIVE! The last thing he wants to hear is how much you miss him and how much you can't live without him! That will just make him wanna come home even more.
I know it's hard for you but think of how much harder it is for him. He has to go to a new place, new home with no family or people he knows and live there for the next 2 years. Share your testimony with him in the letters or emails. Share cool, fun experiences you've had and sometimes share funny, good memories you two have had. If you're ever feeling sad write in a journal and let it all out. Also, PRAY PRAY PRAY it helps so so much. I can tell you I haven't prayed this much in a while. Because he's been gone I've prayed a lot more. And grown closer to my Heavenly Father much more already. You get so much comfort from it and reassurance that your boy is doing good.
Surround yourself with good friends that will have a good influence on you. This one has been the hardest for me because one of my best friends isn't strong in the church and doesn't have the same standards as me. And I had to stop hanging around her because of that. It's not easy, of course, but it will definitely be worth it.
Try to hang around his family as much as you can. Especially on the holidays to show you care. If you can't, try and talk to them as much as possible. That would be bad if you didn't see them the whole two years and then you guys end up together when he gets home and they are like "where have you been?" Ya know?
Turn to your Heavenly Father and family. Those are definitely my two biggest support systems. My family has been here so far with me and support me. I'm so grateful for them, they are amazing!
Date around, I haven't yet because it's only been a little over a month. But I have heard that if you date around it will help a lot. Even if you don't want to, just go and see all the things that, that boy doesn't do that your missionary does. What you love about your missionary and how your missionary is better for you. Maybe just take a really good guy friend out on a date just for fun because you know there will never be any feelings towards it.
Don't dwell on the days he's been gone. Always look forward to how many till he comes home. This falls under the category always stay positive. What helps me is thinking of the lesser number with everything. For example, 2 more Christmas's or in 1 more Halloween for me.
"Missing someone gets easier everyday, because even though you are one day further from the last time you saw them, you are one day closer to the next time you will." :)
Also, get a calendar! It helps so much! At first it might seem like it's gonna be forever but it does help! You can get them at Deseret Book or I'm sure Seagull Book. I made a board that I hang in my room of pictures of him and my calendar, that's by my bed. So I can look out it before I go to bed and when I wake up! :)
Here's some quotes that might help; :)
"The minute you think of giving up, think of the reason why you held on for so long."
"Missing someone is your heart's way of reminding you that you love them."
"We have something special that no one, no distance, no time can take away... We have each other."
"If I could plant a seed every time I think of you, I could walk forever in my garden."
"Remember that wherever you are or whatever you do, your never alone our Heavenly Father is always near you can reach out & receive his aid through pray."
"All trails are to see how obedient we are to what the Lord has commanded us."
"Our greatest glory is not in never failing, but in rising every time we fail."