Wow, Nissala... I'm world-class when it comes to this! I ruminate constantly about everything I ever did wrong, every little embarrassment I ever endured, every slight or situation in which I felt taken advantage of. I just can't turn it off!
I don't know that there is any easy solution for this. I try to practice mindfulness & also what is often referred to as "radical acceptance". When these kinds of thoughts come up, I allow them to come forward. I smile to them, breathe into them, & allow them to fade of their own accord. Sometimes I will place my hand over my heart as a sign of lovingkindness & compassion. As my Signature quote below says: "In other traditions demons are expelled externally. But in my tradition, demons are accepted with compassion."
Trying to ignore, or stuff these sorts of thoughts down not only does no good, it actually strengthens them. However, over time, the practice I have described does help to lessen their impact. It's not a quick fix. But it does work. It is a practice I learned via the writings of the Buddhist nun Pema Chödrön.
My best wishes to you...