I struggled with the same problem for a VERY long time.
Advice from personal expierience : Are you able to do a confession of sorts ?
Can you talk about these regrets with anybody ? A therapist ?
I wasted so much time ruminating over past mistakes etc..... Then I got this brilliant idea , the past is gone , over , done with and unchangeable. Until I was able to be in the present , in the now , I was frozen in time.
Mindfulness helped me tremendously . I started building those new neuropathways that was mentioned. That was the way out for me. Practice it every day. I thought I could NEVER stop that continuous negative " loop of the past tape ". But it is possible. I have been able to start that new way of thinking.
Actually I thought it was some kind of miracle . But I think I reached a certain point where the pain is no longer bearable. So you change or ......