Past is a thing that shapes your present circumstances. Present circumstances and the actions taken in them shapes your future. Thus, an important source of future circumstance also includes those considerations of what have we made out of our past.
Spending too much time in the past results in inaction, feeling of victimhood and vicious self fulfilling cycle of guilt. There are many reasons to not delve in the past more than required, yet at the same time it is necessary to spend enough time introspect to drive meaningful lessons from it.
Spending too little time will let deprive you of the lessons the past has to offer, the lessons which could have let you take better informed and conscious decisions today. These actions would have resulted in fulfilled present and might have secured a better chance at prosper future.
So, the underline principle is to spend time introspect, go as far back as you must to take all the lesson you can; but don't stay there for too long else as it just brings unfulfilled, thus, unhappy life.