You're not alone; it's natural to become extremely emotional while battling any type of cancer. Some people refer to it as a "roller-coaster ride" because there is so much bad
news/good news/bad news.
Unfortunately, often friends and family members who haven't experienced cancer themselves have trouble understanding what cancer patients are going through. For this reason, many cancer patients find it extremely helpful to join a support group. Call the cancer treatment center or hospital where you are being treated and ask for a referral to a support group; they almost certainly have one. The members of a support group will "get it" and give you permission to cry, laugh, and express your fears with more openness than you may find among your non-cancer friends.
It's also possible that your friend is so worried and fearful herself that she wants you to be strong for her. If this is the case, you may need to talk to her about her role in your treatment and how she can be supportive.