why?

there are times I can never understand why I can never let go. why after so long and so much I am still traumatised by what happened. who can help me to sort things out? I want to remove the anger and focus on the love. but sometimes when things happen, the anger just comes back. I know the past cannot be changed. I understand that you're helpless. just as I was once helpless. why can't I let it go? why?