Don't know what to do

I feel like my heart is breaking... I love my  husband.  We have been married nearly 4 years.  both had been married and raised children previously.  knew each other for years (but mostly through others) and share so many great things together.  He is undiagnosed yet, I believe he has ADHD (easily distracted, time management issues, forgets, disorganized-organization, hyper-focus, etc.).  He has so many wonderful qualities that I feel like I am married to 2 different people.  When we are together we get along, share common interests and can work together on projects and things.  When we are out he can't seem to pass up a chance to check out a good looking woman.  If he can get her to smile he says he is just being friendly. When I try to talk with him about how much this hurts me he is defensive and says he isn't doing that and I am the one with the issues. yet he has on occasion admitted he has a problem with this.  It concerns me - the steps he takes to engage a woman... it's more than noticing.   I have found him interested in pictures of women on flickr... lots of them....  Right after we were married I found out he had been maintaining friendships with other women while we were dating - he said he let them know he wouldn't be calling them anymore just before we got married.  I  have discovered on a number of occasions that while he and I are on driving trips he takes unusual routes which go passed or close to the homes of former  "friends".  He had been single for a number of years so there were quite a few.  Some inconsistent stories make me wonder what he does when I am not with him. I work fulltime some distance from home and his work has been a problem due to the economy.  It has been frustrating for both of us.  He doesn't complete projects, distorts what "really" happened, changes his mind and our plans without discussing with me, the list goes on.  I feel like I am running in a non-stop marathon and I know I am not coping with it as well as I could.   I try to understand that some situations are just "ADD" things yet I enable some of the behavior by being passive and letting things go until I can't take anymore.  Other things I refuse to do to compensate.  Then we have an argument.  He says the right things but nothing changes.  I don't nag about unfulfilled promises - just hurt and feel resentment.  I am not sure how to deal with it...  feel so much heartache...  don't know what to do...