I'm not 100% clear on your question but I will try to answer from the way I understand it:
To feel - generally refers to one's attitude or emotion. For example: I feel terrible.
To be - generally refers to a) one's character b) factual descriptions c) things that are more certain. For example: a) I am terrible. b.) I am tall.
Sometimes they can be used interchangeably: I am sad / I feel sad mean more or less the same thing in everyday language.
Sometimes they mean different things: I feel terrible = I don't feel well | I am terrible = I am not a good person OR I don't feel well
Whether feel/be mean the same or different things depends on the context and the words that follow.