Work can be more stressful on some days yet not on others. Is that because of the amount of work, how difficult it is, the pace and deadlines or has any of that got nothing to do with creating stress? Or is perfectionism causing havoc for you at work?

Stress is a physiological reaction brought on by a belief that demand is higher than supply. Therefore the system needs to work extra hard to try to correct the balance by supplying more and faster so as to meet the high demand. This puts stress on the system and the body/mind goes into 'fight, flight or freeze mode'. In this state the body is getting ready for exertion by pumping adrenaline, sending the blood away from the brain and into the muscles to make them stronger for fight or flight. In this state it becomes difficult to think as all the resources are directed at physical survival. 

The belief that you are off-balance is rooted in one's self-esteem which is measured in self-worth. This is often an unconscious belief about one's worth; what you believe about yourself in terms of how worthy of being loved and how good enough or not good enough you perceive yourself to be. If you believe you aren't good enough, this will have repercussions on your belief about your capability and ultimately about the outcome of a situation. 

Hence if you believe you aren't good enough (low self-esteem), then whatever you do (capability) won't be good enough and therefore the outcome won't be good enough. The outcome is often perceived as a catastrophe, with everything going wrong or ending disastrously. 

The world can then appear as a frightening place leaving you fearful, out of control and full of anxiety. 

Feeling 'not good enough' is formed from having unrealistic perfectionist expectations which are impossible to meet. Therefore the perfectionist is always left feeling like they've failed to achieve their goal and therefore end up believing that they are a failure, thus compounding their belief that they are not good enough.

A perfectionist way of thinking is not about  doing things well or thoroughly. It is about thinking in extremes and in undefinable ways such as, " you NEVER help" or  " you are ALWAYS late" or "I will be happy when I'm RICH or THIN or LOOK BETTER". The words in capitals are undefinable and therefore impossible to quantify.  Besides feeling good about oneself is to do with ones self-esteem and not with money, looks or other external factors. 

So the question to ask yourself to check if the above applies to you is: 'What am I not good enough for?'   If you don't feel 'good enough', any amount of demand on you will make you more stressed as the gap between your perceived inadequate capability (supply) and the increased demand widens.

To redress the balance and restore equilibrium you need to understand that you are 'good enough'. This will enable you to deal with any demand in a calm managed way, where rather than feel inadequate you can begin to manage other people's expectations, set boundaries and enlist more help and support from others, rather than continue to struggle with stress and anxiety. 

If you would like to understand your  personal struggle with stress and anxiety, counselling could be beneficial for you.