It's okay to lose touch with former friends (that is, to not feel guilty). This is part of life and I'm always convinced that my friends, whom I lost contact with, will understand as well.

Plus, although we were friends at one point due to common interests, shared environment, etc., we grow up and apart. If chances collide, we will cross paths with some of them.

During my thirties, I felt a bit guilty about not keeping in touch with most of my friends from high school and college. As I reached mid-forties, I have learned to live with the above realization. I think I'd have a good chat with some of my old friends when I meet them by happenstance again.