You're not a terrible person for not keeping up with friends. I've had similar experiences while traveling. I'll make some good friends, and after a few months we go our separate ways, usually 10+ hours away. There's the occasion I'll see a friend again, either in the same or different location, though not likely. I always figure that the quality friendship I had in person can't reasonably be maintained long distance. In those rare moments I do see an old friend, I just resume where we left off. More often than not though, there are a lot of friends and places I'll simply never see again. They have their path and I have mine, it may get lonely, but at least for a moment we could enjoy each others company.