The difficulty of dealing with the absence of an important friendship

I've been thinking about this for exactly two years. It's a constant theme in my mind. I deeply miss a friendship that's no longer as present as it once was, a friend I made my senior year of high school. Ever since I graduated, I've remembered our last conversation that year. It wasn't just any conversation; it was a promise, a promise that we wouldn't drift apart. Well, of course we drifted apart. What haunts me these years is that he's still there, present, distant, but not too far away. I rarely see him in messages, I hear his voice in a sudden audio message on Instagram, sometimes I see him on the street, but never as close as before.

 The group of friends he's part of has nothing to do with him, and that in itself isn't a problem. The problem is that he's young, and even though he feels uncomfortable with this old group, he doesn't abandon it. He stays with it and increasingly distances himself from me and my closest friends (it's worth remembering that they were also his friends in the past). Even though this group I mentioned doesn't like me, I've never offended them or tried to convince them to distance themselves, but I feel like they do this to me personally. What truly haunts me the most, or makes me genuinely melancholic, is that the friendship was never broken; it still exists, but it's distant, cold, and no longer has the same magic it once did. What makes it all a little sad is that, when I remember him, it's living my life, going out with my friends, and consequently, seeing something or being in a situation that reminds me of him, like a band they like but their group of friends doesn't, a concert, or something like that. It's constant, the feeling that, even though he's far away, he's there, his memory is alive and missed, like a brother who lives far away and only calls you at Christmas. It's like that. 

Dealing with this longing is part of life, it's inevitable. I see today that questioning his choices is immature and pointless, but I still find myself thinking about how many good conversations we could have had in the two years that have passed. I miss my friend who became a colleague.