Guilt is not merely a concern with the past; it is a present-moment immobilization about a past event. And the degree of immobilization can run from mild upset to severe depression. If you are simply learning from your past, and vowing to avoid the repetition of some specific behavior, this is not guilt. You experience guilt only when you are prevented from taking action now as a result of having behaved in a certain way previously. Learning from your mistakes is healthy and a necessary part of growth. Guilt is unhealthy because you are ineffectively using up your energy in the present feeling hurt, upset and depressed about a historical happening. And it's futile as well as unhealthy. No amount of guilt can ever undo anything.