Fat is not a feeling. Fat is not a bad word. Fat is a neutral word. Fat is a substance all bodies produce to protect vital organs, to keep the body alive.
Fat does not have any magical power to put you in a good mood or a bad mood, nor does it make someone "good" or "bad".
"I feel so fat" is typically said as a feeling "I feel so..." which typically does come with a host of emotions such as sadness, frustration, anger, anxiety, disappointment, shame, and so on.
Instead of saying "I feel fat" try labeling the emotions you are feeling, remember fat isn't a feeling.
Exercise: the next time you think or say "I feel so fat" ask yourself, what am I actually feeling?
Perhaps it means "I don't feel good enough" "I don't feel satisfied" "I feel like I have failed" "I feel angry, hurt, sad, disappointed..."
Labeling and awareness is the first step to understanding. Get comfortable asking yourself these questions, they will lead you to grow emotionally and help you to understand that feeling fat isn't the problem, the problem is the emotion behind that sentiment and that is what needs to be addresses, not your body.
This is why "going on a diet" will never fix the "problem" your body is never the problem.
Need help digging deeper? Let's connect. This isn't easy work, and it's much easier when you work with a professional ready to help you out of the cycle.