im 16, and i know exactly how you feel, when i was 12 i felt like that too and i was so convinced i was in love i cried when i left the school and really just thought there was'nt any point if i was'nt with him, turns out ir was a crush i was over it in a month,possibly you in love are but more than likely your not. i know it sounds so unkind saying its "just" a crush.but it probably is, looking back now i don't see what i seen in the guy. because although it seemed like more than a crush at the time, it wasn't. your 12, maybe she is your first major crush i don't know if she is, then how could you possibly distinguish between love and a crush. you could feel that way about every girl you have a crush on...you just have to find out
in the mean time,just keep in touch with her when she moves, there is nothing more you can do,time will tell whether the love you have for her is real or not, after a while you may get over her, not forget but just move on i guess,
on the other hand if you don't simple get over her like if after months and months you are still depressed, i'd say your in love ,its rare but their are married people who have been sweethearts since they were 12 . if that's the case then keep in touch, i don't know how far away she's moving or whatever but try to see her whenever you can,if that's possible. if you really are both in love you will eventually be together again, it might be a while but if you love each other you won't be apart for long.
hope i helped and
i hope everything works out for you