I have been diagnosed with stage 4 cancer with an unknown source. But what is the 'etiquette' of trying to priorities doctor's appointments?

Since my diagnosis 2 months, I've been seeing a therapist twice a week to work through the fact I have cancer.  I am also trying to deal with loss of my twin in the summer to cancer. I want to consider my mental health during this time also as I really think it will be a factor in my survival. However, the oncologist needs to find out what type of cancer I have in order to treat it. It has already spread and I still haven't received treatment. My oncologist is trying to book me in asap for MRIs and bone scans to make sure it hasn't spread to my bones. 

I got a call from the Imaging booking office at the hospital telling me they got me in on Friday. I took the appointment. The appointment is 4 hours long because they need to inject the dye into my body and wait for it to seep into my bones before they scan. But, then I realised that my therapist had booked an appointment on Friday during that time. So, I emailed her and asked her if we could reschedule. Her response was surprising. She said 'I don't know how we can make this work'. I wrote her back and agreed to not take appointments during our session. But that was not a good thing for my self esteem or my health considering she has changed the times around a couple times since we started and now I feel like I'm cowtowing to her. 

I have considered stopping with this therapist and taking advantage of the therapy offered at the cancer center.

What is the proper response in a situation like this? How should I prioritize?