Letting Go

It's almost 5 in the morning.
I'm sitting in bed, my stomach churning, because I just saw an old "friend's" website...
This is a guy I haven't spoken to for months.
But I checked up on him, because I was curious.
Why was I curious?
I know I'm not the only person to whom this applies. I know many a victim of heartbreak or harm that still gets hung up on what coulda/shoulda/woulda happened, if only. If only.... Very dangerous words, those two. But common. Regret is a rather vague, paradoxical something. I know sooo many people who say they never have regrets, they choose to live in the moment, etc.
Am I the only one who thinks that's silly?
Regret, like everything else in life, requires balance. It's present to help us learn; too much will poison us. It'd be great not to have regrets...except that I'd never improve or be motivated to change.
I digress.
Anyway, whether they regret past actions or not (in this case, most assuredly not), I can name several people off the top of my head who can't let go of something or someone. Always it's a presence that has proven to be harmful in significant ways. Yet we latch on and wish for the benefits, while trying not to remember the consequences.
It's maddening!
Logically (and this is ironic, considering a conversation I had mere hours ago), I know exactly what I should do and why. And, in all likelihood, I will do that thing. But the infuriating part is that I feel so conflicted and mixed up about it.
What am I supposed to do when I can see things clearly, but the clarity brings no peace?