If you don’t get enough fibre in your diet, your stomach muscles become tense.
When your stomach muscles are tense, your body stops feeling like an integrated unit. Instead, you feel like you have a top half and a bottom half, which are not working together as one.
Tension in the abdominal muscles hampers your breathing, which makes you tired and sluggish.
It also prevents deep breathing, which alleviates anxiety.
All this is cured by having enough fibre in your diet.
You could get enough fibre by eating lots of fruit and vegetables. Failing that, you could eat high-fibre processed foods. (Not particularly pleasant.)
A convenient option is to drink Fybogel from time to time. It’s a “drink” that you make by stirring a sachet of ground Ispaghula husk into a glass of water.