When you have an important ailment, you should search for an alternative doctor. You would not have been betraying your existing doctor by searching for a second opinion. On the contrary, you would have helped him/her provided that he/she has missed something medically.  Receiving a second opinion from another doctor protects you against further dangers especially when you are facing a serious illness which threatens your overall health.
The problems, stress and economic loss which appear in the course of a serious illness, can lead to a trauma. The symptoms of many diseases resemble each other. However, some diseases do not indicate a signal till they spread in the body. Some of them indicates immediate signals as soon as you get the disease. It is of great importance to hold a right diagnosis in the case of a chronic illness and major operation which would affect your life to a large extent.
If you are not getting any better although you have been under medical treatment for a long time or if your existing doctor is unsure about your diagnosis, you should absolutely receive a second opinion.