Mourners can experience rapidly changing emotions, especially in the months following a loss. Researchers tracked daily ratings of well-being of mourners following the death of a loved one (Bonanno, 2009). They found a tremendous variation from one day to the next.
People do not grieve in predictable stages. Bereavement is not one-dimensional. Unfortunately, many people keep trying fit grief responses into neat, predictable, stages.
People are hardwired for resilience and recovery (Bonanno, 2009). The graph (below) shows three of the most common patterns of grief reactions. Notice the bottom two lines; 85-90% of people have a clear reduction in pain/symptoms within two-years. Now, look at the top line. About 10-15% of people continue to experience high levels of grief/pain at the two-year mark. There are many reasons for this.

A Death Loss. Like many others, I did not grieve in predictable stages. There were days, weeks, months that I experienced a wide variety of emotions. If I had recorded my pain on a graph following the loss of my loved one, it might been somewhere between the bottom two lines. But my grief did not start after death and the line would have extended several months prior to the death - when we first learned of the terminal diagnosis. Anyone that has experienced anticipatory grief knows well how painful it is to watch someone you love slowly pass away.
A Nondeath Loss. This loss was devastating (even though there was no death). I was part of the 10-15% that experience prolonged grief. I was in pain for years following this loss - a loss complicated by trauma, ambiguity, and disenfranchisement.
I needed to talk to someone. The problem was finding the right therapist for me. I can still remember the feeling of finally being heard and understood when I sat with my 4th therapist!

Reference: Bonanno, G. (2009). The Other Side of Sadness. Basic Books.

Grief: What the Research Says

Brain Injury Happens to Families