Emotional reasoning happens when you take your emotions as evidence of truth. For example: " I feel like this job will never get done, so it won't." Or, "I feel overwhelmed and hopeless, therefore my problems will be impossible to solve." Or, "I feel angry at you and misunderstood by you therefore you must be an insensitive jerk." Or "I feel like a failure, therefore I am one."
Because you feel things are negative, you assume that they must be negative. And the fact is, emotions are simply emotions. They are not "things as they really are" to quote a very wise man*.
Perhaps you will recognize this phenomenon in your life. Most of us have it to some degree. One of the most common manifestations in my life is when I walk out into the garden and see all the weeds in it. My first reaction is discouragement at the work that will be involved to weed the garden. It feels like a disheartening, never-ending task which makes me feel lousy and want to put it off. But actually, when I finally get past my emotional reasoning and go do it, it's not disheartening work, it actually goes pretty well and I end with a sense of satisfaction.
The challenge is to be able to discern your feelings about things and separate those from your comprehension of things as they actually are.
*Thank you, Neal A. Maxwell