Whenever a horrible thought creeps in to your mind, welcome it on a standstill position. You know how a brave warrior stands still and calculates his options, his moves, his enemy's position, and strategies how to fight his enemy? He is quiet, calm, strong, logical and using his rational capabilities on how to fight his harasser, or worst, his accuser.
With us, with all our human frailties, our worst enemy is the self. We accuse ourselves of horrible misdeeds and submit to telling ourselves, "you're doomed." And the good side within us, called the conscience (soul) is trying to fight what our human self (flesh) has gone through, is going through--harmful and bad things--in its existence.
Forgiveness. When you are on a standstill position, you will be able to think things through-- the pain, the sorrow, the bad decisions, and the failures and mistakes-- rationally. Those past thoughts that make you feel horrible are the means and ways our conscience (soul) is gently and kindly telling us to deal with it, once and for all. And forgive yourself and feel remorse or very sorry for any particular bad (evil) choice we made. And promise yourself never to do it again, or never to allow the reason for the pain to hurt you again.
"Unforgiveable" people are hard to pardon. This is hard to do. But it's the only way to be released of horrible thoughts and guilt. On the bright side, they are the ones that strengthen us to be bolder, courageous and brave to make sure that such bad things will never be allowed to ever happen to us, again.
And move on to a clean slate, so to speak. Try to find joy to erase those horrible thoughts with good uplifting music, messages of hope and of our kind God. You are forgiven, remember?