I'm so sorry to hear this. I hope that I can be some help because I was in this situation at about this time last year. I lost my first love in a car accident, and while we weren't seriously dating at the time it happened, we had just seen each other the night before and were laughing and joking around and talking about the future.
The next day, when I found out, it was like my entire world had fallen apart. I remember just sitting down on the floor and crying, too shocked to do much of anything. And to be honest, it has taken me a long time to even come to terms with what happened. Shit like this isn't fair, and when it happens to people you love, good people who are so full of life and love and have everything to look forward to, it just is completely unbelievable.
A year later, and his death still bothers me. I don't think you ever really get over it, but as a lot of people have said, remembering all the good times you had together is great. At first, thinking about AL at all made me horribly depressed, but I went to visit his grave several times after he was buried and have brought flowers and talked to him (even though I'm not even totally sure what I believe in). His family also did a celebration of his life which was great -- they brought a bunch of friends together and everyone told stories and ate food and remembered AL in such a positive way. Another friend of mine made shirts with AL's face on it and it said "stay stoked" and "keep dancing" and a bunch of people would wear them to music festivals and stuff because they were AL's favorite. Something like that, like organizing the walk, I think that's an awesome way to keep her memory alive. Especially with a group of people who all remember all the positives.
I think I've become more aware now, and it's upsetting that it took AL's death to wake me up to the fragility of life. But everything I do now, AL is always in my heart, I know he's around, cheering me on, and I think that your girlfriend and best friend will always be around you too. You see them in the little things. I still am sad sometimes, it's a grief that's still too deep to be completely gone, but it'll take time.
I know I don't personally know you or anything, but if you need anything, I'm here. I hope sharing my experience helped a little bit. I know how awful this time is and how shitty everything probably feels.
