Part of adult relationships is letting a friend go away for a while and resuming the relationship later when it's more convenient. Having a core set of people in your life is important, but understand that life gets hectic and people get caught up in their own day-to-day bullshit. It's okay to lose contact.
As someone in their 40's I can't tell you how many long time friends I have gone without talking to for years. Then one day I reach out on social media and we have a call or a lunch. It's like nothing changed and everything changed. Suddenly they are showing you pictures of their kids who are now headed to college when the last time you saw them they were starting school.
That's okay, you know? It just happens. When it does you accept it and when you get that meet-up later in life you relish over the catch-up. You get to see how they've (hopefully) flourished. You get to feel their excitement and pride as they share their accomplishments and triumphs. You do the same in return. And sometimes, they tell you about the struggles and you work through a little bit of trauma with them from a perspective that has diverged from their life but is intimate with who they are from the moment you went separate ways. You bring a new perspective wrapped in the comfort of an old friend. Or maybe they do this with you.
What I'm saying is that adult friendship involves a lot of the here and now because our lives get so busy. You start to befriend neighbors or the other parents at your kid's school (if you have any) or the people at your local fitness club. It's because you happen to share at least one thing in common or are just in proximity and enjoy chilling with that person. When you are raising kids, working a job, and doing you daily housework, you really stop finding the time to break outside your bubble and when you do, like to catch up with an old friend, it's a treat.
If that sounds bittersweet, it is because it is.