I expect that a lot of you get those saccharine round-robin things about friendship. I got one once that for some reason struck a chord with me and I have found it very helpful when thinking about friendship. so I share it in that spirit, and if any of you find it too cutesy then stop reading!
The jist is that friends can be for a season, a reason or a lifetime.
Some people are in your life for a season: school friends, ante-natal class friends. You enjoy sharing that time with them, but you both move on
Some people are in your life for a reason: neighbours or work colleagues who help you, who you help because you are there. The reason changes and you no longer have things in common.
And a few people transcend this and you stick to each other for a lifetime.
The message I took is not to get "stuck" or worry when friendships changed, but to think "I'm glad I knew you, but now it's time to move on". Thinking that has helped stop me from getting sucked in to unhelpful friendships.