I have a group of friends with whom I've spent pretty much all of my time during high school and my childhood. We played on the same sports team, spent every free minute playing video games etc. I was never the type of person with a lot of friends, I always preferred few good ones, so basically this group (four guys including me) was my whole social circle for a long time.
Now after high school, all of us started studying. The other 3 guys went to the same town, moved in together into a shared flat and are now studying sports together. I, however, knew for a long time that I wanted to go to a particular university in a different town (about 1.5 hours of public transport time away from theirs) to study computer science.
They don't have much interest in my field of study but that never was a problem at all in our group, I've just been the "kind-of-nerdy" person in the group and never bothered them with these topics. When we played video games together it was mostly soccer, which I would've never played if it were only to me, but I enjoyed it because of them. We've all been studying for three years now and as time passes it seems that they think less and less about me. When at the first year they invited me over to theirs at the weekends, last week was the birthday of one of them and they didn't invite me to the party.
I'm fairly certain that this is not due to them not liking me anymore, I think it's just that they forget about me. Another guess would be that they think I'm not interested in meeting them because my studies tend to be more time-consuming than theirs (especially the first year was) and due to that I couldn't participate in a lot of activities during that period. When I'm in our hometown on the weekends I mostly ask them if they're around too but for most of the time, they tend to stay where they study.
I have no idea how to confront them with my fears of losing my best (and pretty much only close) friends without sounding really weird and "clingy" . I guess the best approach would be something where I don't have to word it but rather show them otherwise, but I can't think of how to do more than asking anytime I'm around and have time. I also would rather know if it was actually more than just them forgetting about me and if they actually have a problem and rather not invite me willingly.
Edit: I'm located in central Europe.