Another big learning for me was that you don't have to be friends with everyone. When I was younger, I really cared what other people thought of me and the worst thing I could hear would be that someone didn't like me or didnt want to be my friend. Now, at this stage of life, things like that just aren't important to me. I'm much more concerned with being the best person I can be and continuing to grow and improve.
Finally, it's important to know when it's time to let go of a friendship. Just like romantic relationships end, friendships can too. People change, that's the whole purpose of life, you may have got on with someone ten years ago that you wouldn't be able to stand now - that's life. Sometimes I look back and reflect on the person that I was five years ago and thing "gosh I was annoying". When friendships end, give yourself time to mourn and be sad about it; after that move on.