You're tremendously grown up about it. I lost my nan this time last year, while I was away at University... Not knowing quite how ill she was (she really was very up and down, and id never seen the worst of it) I didn't see it coming.
Got a call from my dad one day telling me. I was destroyed, and broken by it... You should know though, that knowing it is coming wont make it easier, dont think you wont find it hard, but be sure you have friends and family in support when it does. That is something you can control, and make sure you have your chance to say goodbye. I wish I had... but then, I am so happy I get to remember my nan the last time I saw her she was laughing so much. Ill never forget it.
My thoughts are with you.