Between the ages of 18-24, I lost a child, my grandfather, my closest aunt and uncle, and three good friends(all in car accidents). I'm still recovering. But their deaths made me appreciate every single important person in my life in a way that I never did before. There was a short period of time in my life when I would just move on and away from people, even if we had been extremely close. I had no real rhyme or reason, except for the very act of moving on and away. I figured I'd see the people again I guess. Then people started dropping like flies around me, and I gained an entirely new perspective(and a pretty good dose of hypochondria and paranoia).
My heart goes out to you. It's so difficult to come to terms with the fact that someone who can literally make our entire day, or week, or even give us the feeling that there is reason to live, wont be in our lives ever again in the near future.
I began to journal about my family and friends that had died a couple of years after the last death. I wrote out letters to myself about how much I cared about them, but the most helpful letters for me were the ones I wrote concerning how each individual had literally changed the course of my life. I live each day now realizing that when we love someone or care deeply for them, they become a part of who we are and who we will always be. In that way, they never really leave.