After Joseph revealed himself to his brothers, he said, "Now, do not be angry with yourselves. This was Hashem's design."
These were comforting words, but although it was Hashem's design, they acted as free agents when they sold him into slavery. Hashem could have done it in other ways. There is no way they could escape their guilt. How could they not be angry at themselves?
The answer lies in the word "now." The Talmud says that the word "now" denotes teshuvah. Appropriate repentance for the wrongs one has done, by resolving not to repeat them and by trying to eliminate from one's character those defective traits that made the wrong deeds possible, can lift the heavy burden of the past off one's shoulders and allow one to deal with the "now," with an unencumbered present.
If we do proper teshuvah, there is no need to be angry with ourselves for our mistakes.