Hi, I'd like to help you with your question.
It is very understandable that you have developed this fear. You are watching your best friend suffer and face death with a terrible form of cancer. Being told you are going to die at the age of 30 is one of the worst traumas someone faces. Being with her, you are going through the pain, sorrow and other emotions of the situation as well. This can cause extraordinary stress and you are reacting to that stress by developing fears.
Adding to your friend's situation is the fact that cancer and the fear of cancer is everywhere. There are breast cancer months with pink everywhere you look, famous people dying of cancer and news stories about cancer. With the frequency that we are exposed to the thought of cancer, everyone feels their chances are extremely high to have cancer. Now your friend as it and the reality of cancer is impacting you directly. The feelings of helplessness and hopelessness can cause you to feel you are next, when in reality you are just fine.
To deal with your fear, first you need to understand it. Learning more about how trauma can impact you and the effect on your emotional wellbeing is the first priority. The more information you have, the better you can confront your fear and cope with it. Here are some resources to help:
Also, look at the facts. The chances of you getting cancer are small. Most of the breast cancer diagnosis are of older women who are well on in years. And even if you did get cancer, you have an excellent chance of recovery. Treatment is improving constantly for all types of cancer. Your friend's experience is rare.
Seek out support. Talk to others about your fears. Many people fear death, but they also weigh the chances of illnesses like cancer with the facts. They can help you find new ways to look at the situation and see that while cancer is common, most people either do not get it or are cured of it.
Talk to a therapist. Even if you did not have a fear of cancer, just going through your friend's illness is traumatizing enough for you to benefit from talking to someone. The therapist can help provide support and guidance on how to overcome your fears and enjoy your life again. To find a therapist, talk to your doctor about a referral. Or you can search on line at http://therapists.psychologytoday.com/rms/.
I hope this has helped you,