The thing about the past is that you have to let go of the bad whilst hold onto its good memories.
Holding onto something that's past, whether it was good or bad, is crippling. The wisdom is in recognizing that it's passed. To allow the present and future to mature in and of themselves is to quit living them in the light shadow of happy times or the darkness shadow of sad times.
However, releasing does not imply forgetting. Being able to forget the memory of a past that did us bad is a form of liberation from that past. Holding onto this memory breeds resentment. This does not mean though you should completely efface your memory; hold onto those that bring a smile to your lips. It is the human condition to struggle remembering the good times where the bad times seem emblazoned onto our consciousness, and for this reason it takes deliberate effort to focus on happier times. Ironically, these memories are the ones that enhance our ability to be happy in the present.
Part of living life beautifully is treasuring its moments that gave us a meaning and a joy to be alive.