If something didn’t work, if something went wrong, if you didn’t get the result you were hoping for… Why? In 3+ years of research, not everything can go perfectly. What’s responsible? Who is responsible?
At every stage, but particularly at the end of the PhD, you have a choice in the story you tell yourself. You can say that it’s all your fault. You can say that things were beyond your control. Or you can change focus: treat everything as an opportunity to learn. This happened, why, what next? This happened, why, what do I do differently?