its great that you care about this girl so much..but you have to remember..you are 15! do what KIDS your age are doing. you shouldn't be too worried about marriage right now. sure, you're allowed to fantasise about the idea, but clearly you are not mature enough at this stage. go on dates, study, travel, etc before you think about being tied down. please dear heaven, dont tell me you want kids soon too.
you might be feeling like you miss your girlfriend a lot if she's your first love? i dont know, but it sounds like a 'first love' feeling to me. just enjoy your teenage years kiddo..you've got the rest of your life to think about marriage and the lot.
"We all take different paths in life, but no matter where we go, we take a little bit of each other everywhere."