Do you enjoy reading? If so, check out Radical Acceptance.

I'm not sure I could pinpoint the difference between regret and grief. Maybe you can have regret without grief but for me I've not experienced grief without regret. Maybe regret is for (as you say) the things that were actually under my control that I theoretically could have done differently.

At the heart of it for me has been talking about what I regret, trying to understanding what X regret was about for me at the time, why it made sense to me to do X. But even though I'm nowhere at the end of the process, I find accepting that and why X happened is what releases some of the regret's talons over me. Beating up on myself, blaming myself, having little empathy or compassion for what I did and why, those are all things that seem to be tempered by a realistic "this is what happened" and sharing what happened, having my T be a witness to my attempts to accept it.