Friendship troubles

So I have a question for anyone that may have a negative friendship in your life, either now or previously and what have you done to help it to either not effect you anymore or to remove it? I have a friend that is very up and down and when she's up she's the greatest friend, truly, but when she's down, twists everything I say around so that I have to choose my words very carefully and walk on eggshells. It makes me feel like a horrible friend and I know in my heart it's not the case.