There are times when I feel not regret for my decisions as they have led me to where I am today, but some melancholy.
In such cases, sometimes I let my emotions flow through the light of my consciousness. Other times, I ask myself pointedly if these emotions compel me to act, then what are these actions, and finally will these actions resolve the cause of the sadness. Still in other cases, I have a "conversation" with myself posing as a dear friend to aid...
@DoubleD - I tend to look at things from an evolutionary psychology point of view. It makes sense to me, personally. And in keeping with that there's a purpose, a proper time and place, for every emotion - even the ugly ones.
I'm comfortable with melancholy - sort of at home there. 
When it comes to decisions and regrets... I strive to have more than one reason for doing anything. It makes life feel more efficient. For every action I get to tick more than one box. 
@DoubleD - And if one reason should turn out to be faulty, flawed, or otherwise not applicable... well, at least there are other reasons to support that action I took. And this helps to minimize regret for me. 🫡
And of course there are other ways to help stave off regret: maintaining a sense of personal honor (essentially keeping a moral code and not violating it. Though it may differ from the moral codes of others, consistency is key). And reminding myself that I can only control myself.
