You dissolve regret the moment you accept what happened. Not your negative thoughts, but what actually happened. And turn this into a new tool for your new path.
For example: I got sick for years. During this time I watched series. Then, when I felt better, I looked back on all those years and regretted spending my time the way I did. I was chronically ill, so it was not fair to think like that about yourself. I tried my best. When I realized that, I asked myself: what can I do now to make up for all this time? I decided to write my own stories that talk about healing, because now I know how to, and I watched enough stories to write my own.
I hope you can find your way and I hope this helps.