how to keep friendships afloat?

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hi yall,
so i entered college 3 years ago as a shy, self loathing dude. Despite that i forced myself to be extra sociable in the first days and actually managed to meet people. Eventually over the semesters we grew in a tightly knit group of friends. Some new people came in, some went away but this group kinda stayed the same. In these 3 years I became more and more social and actually enjoyed each day.

But now we're at a point on our cursus where we're either in internships or abroad. And my old habits came again. It's been 2 months since I saw them irl, and it made me feel real lonely. That in turn pushes me to isolate myself and being mostly silent in the different group chats. It's the usual cycle, you isolate yourself and you get upset cause you're lonely (fucking dumb, right). Anyway as time passes I feel as if I'm drifting away from them and it's more and more difficult to talk to them. It seems kinda out of place to just come back out of the blue an tell them "I was doubting your friendship and didn't talk to you but what's up!". Feels cringy and selfish.
Sorry for the long text, but i needed to vent somewhere.

tl;dr : i used to be surrounded by friends and now feel disconnected.

What are your advices for dealing with loneliness/keeping contact with friends?