It is a bad thing to hold onto the past, mostly because you are attempting to hold tight to something that is gone, and can never be regained by you. Even if the nature of it was predominately characterized as ‘good’ (or perhaps, especially if it was good), you are setting yourself up for disappointment, at the very least, by not letting it go and moving forward, in a contemporary sense.
Remember the past… and if it was a good time of your life, cherish it- you were fortunate. But do not hold onto it… its time has come, and gone. Don’t corrupt its memories by trying to live in them forever