Learning from the past or leaving it behind?

Why is it so difficult not to look at the past? And is it necessary to look at the past to understand and prepare for the future? Good medicine has always been to be positive and look ahead - not backwards, but how do we do that? When we look back, we do it either to blame ourselves or someone else because it is easier. It is easier because then we don't have to take responsibility either for our actions or for what is going to happen in the future. Problem is, it obviously doesn't work. In the end of the day only WE can take responsibility for our lives and indeed be responsible for our actions. But it is hard! It is hard to find the border between leaning from the past and blaming the past. When is it a lesson we must carry with us to the future and when is it a mistake we must leave behind? Having said that, are there such a thing as a mistake or is it all lessons that are opportunities to learn from?