Friendships are a balance. There are times when you are needed to give support and times when you need to receive it. You might think that toxic friendships are easy to spot, but that isn't always the case. Someone you genuinely care for, and who might genuinely care for you, may seem worth the emotional upheavals of a turbulent friendship. However, it's important to recognize the point when a friendship causes you more detriment than happiness.
There is a difference between supporting someone in a difficult time versus sticking around in a situation that causes you pain. Are you in a friendship where you are constantly bombarded by negativity? Does your friend monopolize your time when they need you, only to be too busy to ever return the favor? If that friendship sounds like yours, you'd be right to reconsider it. Friendships are supposed to help raise you up -- not drag you down.
You might feel guilty just for thinking about ending a toxic friendship. Emotional manipulation is a classic sign of a toxic friend, so they may be great at unjustly making you feel bad. The important thing to remember is that you need to look after your own well-being. You are not a bad person for leaving a toxic situation.
Another reason people continue toxic relationships is the sunken cost fallacy. For example, you've been friends for so long that you think it isn't worth ending a six year friendship over a few months due to a few issues. People tend to think that they need to stay in a relationship with someone for fear that they've "wasted" all the time that they've been friends. The truth is, that time with someone is never really wasted. You will always have memories, good and bad, and you will always have learned from your experiences.
Ending any friendship, toxic or not, is difficult. Be honest and firm with the person. After that, you will be free to enjoy a more drama-free life, with those who truly care about you.