Personalization is when you assume responsibility for a negative in your life even when there is no basis for doing so. For example, your coworker, whom you've been mentoring, fails to receive any recognition awards at the department banquet and you feel guilty because of the thought "It's my fault that he didn't earn any recognition. It was my responsibility to see that he earned it. This just shows how I failed."
Another example might be one where you invite your boss and his family to dinner at your house on a Sunday evening and and at the last minute they call and cancel because of illness and your brain instantly tells you, "They cancelled. What did I do to foul things up?"
Or your eight month old wakes up three times each night for weeks on end and you hear, in your head, "She isn't sleeping through the night yet. I'm such a lousy parent."
Personalization causes you to feel crippling guilt as you confuse influence with responsibility for and control of outcomes. Certainly there are things that you can influence in the lives of those around you, but in the above cases you have confused that ability to influence (or not) with your control over others. And, you don't have control. Nor should you. Ultimately every person you know holds the secret to the key causes for their own actions, not you.
Have you ever heard yourself speak to yourself in this cognitive distortion of feeling that the failure of someone else or some project you are involved in means that you, personally, are a failure as well?
Most of us have at one time or another.