You might not let go of the past because you are stubborn. The stubborn aspect of your character would mean that you would consider it importnat not to ignore what has happened before thereby entrenching yourself in past mistakes or glory. This prevents you from moving forward as your character will always see new experiences in the light of previous experiences.
You might be inflexible and unable to move beyond the past. Your experiences might have made you what you are and the characteristic of inflexibility would mean that you could not let go.Perhaps the characteristic of intolerance would lead you to not want to leave past events behind you as well as be unwilling to accept different views from yours. Certainly a level of unwillingness is suggested in not moving forward from the past.Submissive people might be led by others to not let go of the past.People who are not free of others, both physically and emotionally, will find it difficult to be free of a past.People who are unforgiving might hold onto the past.Vulnerability can lead to a holding on to the past.