You and I have very similar situations. While I haven't had a girlfriend die (I'm terribly sorry), I have lost 3 girlfriends to my still best friend. While he has since changed (he found jesus or something, I don't even know), my acquired tendency to worry hasn't. I am going to Boston College this August, while my girlfriend (6 months) is headed down to Delaware.
I worry about her constantly. She loves drinking, and shes going to be 5 hours away from me. She'll probably cheat on me right?
Well, while I worry every day that she'll cheat on me, I miss the obvious (and I should take a dose of my own advice here): she loves me, and has stayed with me for 6 months, and has promised me that she'll stay with me in college. I almost miss the fact that when I go to leave her house every time I'm there, she wraps her arms around my waist and falls to the floor, trying to keep me at her house. An originally prudent girl, she gave her virginity to me recently. What you have to do is look at the signs. They will tell you exactly what she intends to do. If your girl is acting anything like mine, she wants to stay with you as much as you want to stay with her, no matter the distance. If you have faith in her, and give her the benefit of a doubt, theres still a chance she'll break your heart. But if you don't, then there's a 100% chance neither of you will have a happy relationship with the other one. I know its tough, I myself can't even believe im typing this to you, but the fact that I am means it must be true.
The only thing I worry about is her getting shitfaced at some party and doing something that she'll regret. But her character would never allow her to do something like that to me intentionally, and she loves me so much, I'm sure shell safeguard herself in environments like that.
Anyway, I hope this has helped.