For most perfectionists, there is some sort of rule or assumption underlying their need to be perfect. Each rule or assumption has its own particular variation. Most, however, take one of the following forms: "I have to be perfect or ...." "If I'm not perfect then..." "If I make a mistake it means..." "If I allow myself to accept anything less than perfection then..." Notice that each of the previous statements suggests the presence of some sort of fear. "If I am not perfect then..." I must be a failure. Or people won't like me. Or whatever. "If I allow myself to accept anything less than perfection then..." my life will turn into a complete mess. Everything I do will be mediocre. I'll never be successful. Or whatever.
In this way, perfectionism serves to protect a person from his deepest fear. Usually the fear is rooted in the belief that one is not good enough as he is. If he is perfect, however, people will not see how inadequate he really is. When he is perfect, he can forget about his own perceived inadequacies, at least for a time.
Oh, but when he makes a mistake...When he makes a mistake, he becomes overwhelmed with the knowledge of his own deficiency. He berates himself for erring and expects others to do the same.
Maybe perfectionism has served him well in life. Perhaps it has garnered him praise and recognition. Perhaps he has achieved great things. Would this have been possible if he hadn't demanded of himself perfection?
Perfectionists know well the consequences of their unattainable standards and relentless need for perfection in all things. Yet because perfectionism can be advantageious at times, there is quite often ambivalence about efforts to change. Thus, the first step in "challenging" perfectionism is to make a list of its advantages and disadvantages. Complete a thorough cost-benefit analysis; what does perfectionism cost you and how do you benefit from it? This enables you to fully explore your ambivalence. In most cases, it also helps to increase motivation for change.
So what about the benefits of perfectionism? How do you let go of something that serves you so well in so many of your endeavors? Ask yourself this: Is perfectionism the only way to attain these benefits? Can you still be successful if you set more realistic standards and become less self-critical? Will people still like and respect you if you're less than perfect? Will you still do good work? Will things still get done? Or will you become just another mediocre member of society who never accomplishes anything notable?
There's only one way to find out; do it and see what happens. Develop and carry out a few "behavioral experiments." Start small. Pick some small job you are tasked with completing each day. Most likely, you expect yourself to complete even this small task with 100% accuracy. Determine what this (100%) looks like. Then decide what 90% looks like, 80% looks like, 70%, and so on. Designate a specific period of time -- say, three days -- and resolve to perfrom the selected task with 80% effort and accuracy. Pay attention to what happens as a result. Do people berate you? Does anyone even notice? And how do you feel? Do you feel more or less stressed?
Alternatively, you can try intentionally making small mistakes and observing the outcome.
What you're really doing is testing your assumptions. Remember, perfectionism serves as a defense against underlying fears of inadequacy, rejection, etc. The goal of your 'behavioral experiments" is to see if what you fear actually comes to pass when you intentionally perform in a less than perfect manner. (Spoiler alert: What you fear almost never comes to pass. Although you might want to enlist the help of a good therapist to ensure that you've selected appropriate behaviors to adjust).
Hopefully, these strategies will get you started on the path to change. Keep in mind that old habits die hard. Change is never easy but it's always possible.