On of the most similar thing is our PAST.
Its might be good or bad. We all some good memories or some worst memory ever.
Few people can't forget or regret from their bad memories or past. Who always think about his/her past or unintentionally remind that.
If you think about your bad memories repeatedly you can't stay happy from heart and soul. You can't rid from this. Even many cases I have read before they become mentally sick . Because they face a lot of pain for his/her past.
Most similar incidents:
I researched on it and I find that most of the people can't get rid from thei first love. They unable to forget about that person. Because their love fit a like tree in their heart. Its unforgettable.
Other thing is Parents Deaths. The people who see their parents death in childhood, they spend their whole life without parents. They less from parents love forever. They also stay in depression from their worst past.
Many child faced sexual harassment. Almost every child faced this from his own family members. They victim and the person who abuse. Both doing that unintentionally hormones proby that I think. Some criminal are doing this from their habits. Some child can't forget the memory and suffer for a long time.
How to rid from bad memories?
My Advice:
Keep smiling and share your good memories with your friends repeatedly.
Don't stay alone make you new Friends and make your own Human Network.
Invite your friends every week at your home its might be expensive for but not more than your happiness.
Never get angry with anyone love your enemies too
Just move on. Forget him/her