The more things change the more they stay the same

What am I doing?  I'm filling a void.  And so is he.  My exhusband is in a relationship with the woman that he cheated on me with for years.  Now she's a full fledged girlfriend.  And he's cheating on her with me.  Really?! It sound ridiculous to me.  I am ashamed.  And I am not ashamed of much.  I enjoy spending time with him.  And at this moment he's interested.  He's finicking and I figure soon enough the interest will fade or he and his gf will make up.  I'm so disaapointed in myself.  But another part of me is like why not.  I'm not seeing anyone right now and this fills some lonely moments.  I'm fine as long as I don't go back to where I was.  The problem is I have to keep my distance.  I choose to lie to him which is not my way.  I won't let him get over on me....not in a way I don't want him to.  And so to make it seem like I'm sweet and cool and caring.  The person he knew me to be and but somehow easier going...I just lie.  He asked me to borrow money and I say no.  But then he didn't have gas and I wanted to see him and I said I would watch our kids.  And he had already dropped them off.  So if I didn't help himget gas then I wouldve been stuck with the kids and not had company which was the only reason I agreed. 
So I gave him the change from my jar and change purse.  When I really had more money in my acct, but didn't wanna give it to him.  I know he's not in love with me and I'm not even sure if I'm in love with him anymore.  But I am settling.  Is settling going to block me from getting who I really want?  That's my fear.  And that I 'll fall for him again?  Assuming of course that I'm not still in love with him and just deluding myself. 
I don't think there's anything wrong with people choosing to spend some time to fill there loneliness as long as everyone is single.  He's single, meaning not married, but in a relationship.  I feel that this is out of my character, but because of who it is I don't feel a great deal of remorse. 
He's sometimey, so as long as I don't chase him, it should be a wash pretty quickly.
I'm moving on in so many other factions of my life, why is this one so hard to shake loose?