What's to say that the students who forgave themselves didn't do so as a result of some other underlying reason that happens to be correlated with a propensity

What's to say that the students who forgave themselves didn't do so as a result of some other underlying reason that happens to be correlated with a propensity to forgive oneself? At the very least, one could argue that inability to forgive oneself could in some cases be indicative of, say, depression. Perhaps self-confidence is simply what helps one succeed?