Why do some ADHDers lie so much

The lies and deceit kill me.  And some of the things he says.  Its incredulous that he thinks I believe all the stuff he says.  And sometimes he lies to other people in order to cover up a mistake.  Its like he is trying to project an image of being perfect. 

I have been handling so many things wrong for so many years.  Now I am trying to do things better.  And he will notice and comment.  His behavior will improve for a while, then it's back to the same old thing.  I am doing a marriage program I saw online.  The advice is that when you are at the point you think you're done, give it another year.  There are all these things you do in hopes that your spouse sees the changes and adjusts his/her behavior therefore making the marriage better.  I don't know that this will work with someone who has ADHD.  There is no consistency.  He sees improvements, he acknowledges my improvement, and he changes for a few days.  But it is only on the surface.  The woman friend that he told me he was going to end a relationship with (emotional not sexual) is still in the picture as much as he tries to get me to believe that she is not.  I want to take a hammer to his bleeping cell phone.  If you don't have anything to hide, why is it that any time I am around, you turn the phone over to face down? 

I posted here last year about the lying and the emotional affair being related to ADHDer impulsive behavior.  An ADHD person responded that it was not impulsivity but it was selfishness that causes him to act this way and that until he hits rock bottom, he won't change.  Well a few months ago, he was intent on taking his own life after he found out I had a long discussion with the other woman.  He had been lying to her too (shocker).  And we both let him have it.  He then decided to try to use suicide as an escape route.  But the night before his attempt, I took all the guns and locked them and took the bolt from the rifle and locked it up too.  After I got the email that contained his intent to harm himself, I called the police who tracked him down (thank God for Onstar).  He was taken to the hospital but by the time I got there, he had talked his way out of being admitted (yes, he convinced a trained professional that he was not a danger to himself) and was released.  Luckily, this slowed him down enough to realize taking his life wasn't the answer.  But you would think that this was rock bottom.  Nope, still lies and still has contact with this woman.  After all this, why should I expect he will ever change?  He is on meds (when he remembers to take them) but not seeing a counselor.  We tried counseling but that was a nightmare for him and he won't go back.  So I went alone for months.  I stopped going when I thought the emotional infidelity was over because I can deal with the other aspects of ADHD.  I know the "affair" isn't over so I have been trying everything else to get this marriage to work.  I went back to the counselor and will probably go again soon.  Try as I may to change, that bleeping cell phone is a trigger for me and I get angry.  Especially when I am doing all the right things.  I don't think I can make it a year unless he changes and it consistent with it.