The feeling of hopelessness is the worst one for me.
It's an emotion that leads to others and amplifies them.

I must agree with KDigityDog
Helplessness
....
When the person who was your best friend suffers and weakens, then he stops recognizing you and slowly disappears from this world, and you can't do anything about it.
I had this feeling for the last 4 months of my dad's life. (brain neoplasm)
Fortunately, on the last day of his life he regained consciousness for a short time, and these last hours and conversations with him turned out to be one of the most beautiful moments in my life.

i think guilt is the worst one to have because its just horrible to have and sometimes not fixable based of what youve done.