I always disliked the feeling of being irrelevant

I can't consider an emotion "the worst" because they all matter and they are what makes you... YOURSELF!!


For example, after sadness allways comes happiness, so sadness is not bad, It helps enjoy the moments even more...




I don't know if I made myself clear

Realising that you spent too much time feeling sorry for your self instead of improving on what you lack.
Or regret in general.

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.