I'm realizing, more and more, that people break their promises. Maybe it's not always intentional, but it happens, and it still stings the same.
So never promise you'll always stay. Don't let her think that you're here for the long haul. Or that this friendship means so much to you. Because when you leave, you'll just break her heart, and you'll be like everyone else.
Don't pretend to spare her feelings.
And don't make her promise things to you that you will never keep yourself. Don't tell her she shouldn't cut people off, if you are just going to confirm why she should.
Don't tell her you can't imagine not having her in your life, if you can turn around and walk so easily out of hers.