Hugs
In my case I built a core belief to replace the one I didn't want that said basically that none of that (the past) is important to function. I had a traumatic experience that sent me into a tail spin of perfectionism and doubt about my own skills. On top of a core belief built on others comments this core belief and my actions became quite destructive to my life and how I treated people. I became so busy trying to prove a point that I missed out on life itself. In short I became some one people didn't want to know because they didn't know how to take me. I was boring. This made the core belief seem even more real. It took someone to tell me the core belief was wrong since it was someone other than me that installed the negative one to start with. I only built on it making it stronger.
Remember the three parts to building a core belief. Perception, attitude and belief. I can see where the negative one is wrong, I want to change it to a positive one, and I believe not only that the negative one is wrong but that I can build a positive one that will change me and it.
In my case what I want to build is that all that pain in the past is not important and will only cause more pain. To do this I have to focus on only the good points from the past and every time I think on the crap I have to say "not important" so I will believe it because it really is not important since it can not be changed. It happened but that does not make it important. Most of us have had bad things happen to us and have done bad thing too, some times in retaliation. No matter what the reason there is no way to change this other than to let it go with the core belief that it is not important to functioning. Feeling mad or bad are just negatives and destructive and time wasting.
I know from past experience how hard it is to let go of the past but one must even letting go of good thoughts if they are becoming destructive by making a person think they can go back again. Times change and I must change with the times.
I know I had some very good times but I also know I can not do them again, I'm glad I had them. I had some bad times and I will do every thing possible to not let them happen again but the thoughts are not going to control my life. New core belief, "they are not important for me to function".
Now that is just the main core belief I had to build, there are a number of smaller ones related also such as "I am capable" to counter the one that says I'm not. And "what I am doing is important". And "I am not wasting my time even if it seems so".
There is no magic core belief that works for all negative thoughts. Each person has to recognize the negative ones or in most cases have someone point them out so they know what to build to counter them. But Accepting that very little from the past should influence the present and future is a good starting point. No one that I know has had such a messed up past that they can't find something to counter the negative replays.
That said, I do know how hard it is to not let the negative run on and on. You see it is part of survival. Replaying it keeps you from redoing it or at least it is supposed to. So here a person needs something like " okay I know, now go away". Like getting rid of a song stuck in your head, think of something else, something positive, something good. And don't forget the "It's not important" core belief.
Davit