what is the emotion of "shame", really?

especially toxic shame?

what is the difference between it and sadness (or hurt)?

is it a real, actual emotion? or a concept? does it exist?

and i can't differentiate between the concept of shame, and fear sometimes (often).

what is it? and is there a way to know if i or any of my parts is "feeling" (or experiencing) it? (if it exists). is it an emotion, rather than a concept? or not?

and how to differentiate that from "fear" behaviours? or should i even?

and i don't know if all "hiding myself" is out of fear or "shame". or is it "fear of shame"? what is shame, even? i cant understand or tell.

and if it exists, is it a primary or secondary emotion? most of the time at least?