Psychology
There is a client in every operator’s book who gets under the skin in a way the others do not. Same scope, same money, same email cadence, completely different reaction. That difference is not about him.
“Everything that irritates us about others can lead us to an understanding of ourselves.” Carl Jung
Jung called it projection. The parts of yourself you have refused to look at do not disappear. They get assigned to whoever is standing nearby. You do not experience them as yours. You experience them as that guy.
In business this is expensive, because the emotion arrives wearing the costume of professional judgment. He is disorganized. She is unrealistic. They do not respect the process. Sometimes that is true. Sometimes you are looking into a mirror you did not know was there.

The test is simple and it is not comfortable. Ask whether the irritation is proportionate. If a client is ten percent late and you are ninety percent angry, the extra eighty is yours.
Run it the way you would run a variance report. Reaction size minus event size equals the part that came from inside the building.
The three that come up most
The one who wants everything cheap. If hagglers enrage you, look at your own pricing. People with clean numbers do not get angry at a lowball, they get bored by it. Anger usually means some part of you also suspects the number is soft.
The one who will not decide. If indecision makes you furious, check where you are stalling. Founders avoiding a hard call of their own have almost no tolerance for watching somebody else avoid theirs.

The one who takes credit for your work. That is the hardest, because the honest version is that you want the credit too. Wanting it is not a character flaw. Pretending you do not is what drives the resentment underground, where it comes back out as a passive-aggressive status update.
A reaction out of proportion to the event is a message with your own return address on it.
What to do with the reading
Projection is diagnostic, not exculpatory. Some clients are genuinely bad clients. The point is not to absolve everybody, it is to stop confusing your material with their behaviour, because those two problems have completely different fixes.
If it is theirs, the fix is structural. Tighten the scope. Change the payment terms. Fire them. Do it unemotionally, because emotion in that decision is exactly what makes operators keep bad accounts too long and drop good ones too fast.

If it is yours, the fix is internal and it is fast. Name the trait. Ask where you do that. The irritation usually drops within a day of an honest answer, and the same client becomes workable without changing a single thing about himself.
The compounding version
Over enough accounts this turns into a real edge. Most of the market is filtering clients through unexamined reactions. They lose good accounts to their own blind spots and keep bad ones out of guilt. You want to be the operator whose client decisions run on economics, having already paid the internal cost somewhere else.
Marcus Aurelius was doing a version of this every night. He was not journaling for posterity. He was auditing his own reactions before they hardened into imperial policy. The man ran an empire and still decided the first thing worth governing was his response to an irritating person.

You cannot outsource judgment to a nervous system you have never examined.
The practice takes about four minutes a week. Friday, list the three people who annoyed you most. Write the trait beside each one. Then write where you do that.
Most weeks one of those second columns comes back empty and the other two come back full. The empty one is your actual client problem, and now you can price it, restructure it, or end it without flinching. The full ones were never about them.

Your reactions are data about you before they are data about anyone else. Read them in that order.