Tips on Maintaining Friendships?

Hi ladies! Something that has been bothering me lately is feeling guilty for not maintaining friendships well with those who aren't physically near. It's hard because I know so many people now. I have my high school friends, college friends, friends from my yearlong volunteer program, and I'm headed to grad school so that'll be another group of friends. How can I keep present with so many people? It makes me sick thinking about how close I was with someone and now how we barely talk just because of distance. I try my best to text people and inquire about their life, but it's hard keeping up. Are most friendships not supposed to be long lasting? Am I being a bad friend? Anyone else in the same boat?