To ask for forgiveness or not?

So the big question I have at the moment is when is it healthy to ask for forgiveness and when it is selfish?

So there was this guy...when I was 18.  There was like 6 months where we basically used each other to satisfy our physical needs.  I ended it.   And the thing that haunts me is that i let him carry all the guilt and responsibility for what we did.  He apologized to me later and I forgave him, but I never acknowledged my wrong.  My responsibility.  I never realized fully until today that I used him too.  And worse that I utterly abandoned him, possibly in a time of need, just because I was done with the physical stuff.  I wanted a real relationship.  I wanted to be a good-girl again.  I didn't even think twice about his need, his hurt.  I never asked what pain he was running from when he called me into his arms.  I don't really feel guilty about the physical stuff.  We were 18.  We were exploring.  It was fun. But I feel enormously guilty for how I abandoned him in the end.  In all his hurt.  In all his need.  In the middle of his suffering, I said I'm done.  I bailed.  I moved on and built a happy stable life for myself and didn't look back.  I know he is now married and has a kid.  So in many ways he has moved on now too and built a happy stable life for himself.  So the question is, is it selfish of me again to want to apologize now and ask for forgiveness?  It would make me feel better.  It would relieve so much of my guilt.  It would free me from wondering how much pain and hurt I caused.  I could let it go and know for sure that he is okay...even on the inside.  I need to know he's okay.  But what if he is okay and my coming to apologize brings it all back up again and creates pain and trauma all over again?  Can saying "I'm sorry" do that?  Maybe.  It's not like I can fix it now.  I don't know.

Then there is the other girl.  She loved him.  She had dated him for 2 years.  She was also having a physical relationship with him, even though they were broken up.  We were friends, good friends.  I didn't know, until I caught them.  I should have ended my physical relationship with him in that moment, but I didn't.  We both continued using him, and him using us.  In that moment I betrayed my friendship with her.  She talked to me about her relationship with him and I didn't tell her that I was with him also.  I kept it secret.  I felt like it was my turn.  I had started to have feelings for him again. I didn't tell her.  I still haven't told her.  She's also happily married now with two kids.  Not to that guy.  We both ended things with him eventually.  But still, this haunts me too.  Because, well...that's freaking Horrible!! I mean.  Come on.  This is bad stuff.  I dream about her often.  I really want to come clean with her.  Tell the whole story.  Put it all out on the table.  But maybe that's selfish.  Maybe I should carry this darkness so she doesn't have too.  Sometimes when spilling our guts we feel better, but the other person feels worse.  I'm sure that would be true in this case, but maybe not.  I have this tendency to feel that openness is the better course to take.  I hate secrets.  I feel like sometimes they are a necessary evil, but still openness is the ideal.  Is that naive?  But what if I'm withholding information that she needs for her wholeness and wellness?  How do I know?

What do you think?  Would you apologize and confess or would you keep it to yourself?