It's always hard, it always hurts. Whether it be a friendship or a relationship, or even a family member that walks away, it's hurtful and extremely difficult to let them go.
My entire life, I've been the supporter. I've been the one to try to save the world for everyone, no matter how they've treated me in the process. As I've grown into an adult *WHAT WHO SAID THAT STOP* I've realized that no matter how difficult it may be at first, letting the people go who are no longer contributing anything positive to your life is HEALTHY. It doesn't make it much easier - and it always stings at first, especially if the other person is the one to turn away first. Just knowing that it's what's best for me, though, helps.
I saw a quote the other day that said, "I've never lost a friend in my life." Think about that for a moment. No matter the circumstances - whether they choose to leave on their own or you have to give them a little push - those who exit your life aren't meant to be there. Your friends will stay, and you'll be better off.