Open to any thoughts/perspective on dealing with regret of past decisions/actions and the associated pain/grief of thinking about "what might have been". While I understand we cannot change the past, acceptance of that fact does not result in forgiving oneself for doing something that is life altering (or at least seemingly it is).

What's your background? Not being flippant, because it makes a lot of difference. The Christian formula for this is confession, asking for forgiveness, providing restitution, and then doing your best to do better in the future. I've never found a better way to handle these things

Speaking as a Catholic Christian, "regret" can be used by evil forces to keep you in a state of "potentiality" instead of becoming the person God envisioned you to be.

Maybe you have heard of "Catholic guilt" because we make a point to acknowledge our sins every time we go to mass. We do this, because we know Jesus has died for our sins and we can move on because we have been redeemed.

If we stay in a state of regret, we can risk denying the saving grace of God offered through the sacrament of reconciliation that is intended to make us whole again before God and the community of believers.

We should be remorseful but do not dwell on any past sins that have been forgiven. HTH

ETA: remember that we must love ourselves in order to love others.