40 and married (i.e. divorced) twice, with plenty of sex partners *during*? Okay, so that means you cheated. You sound like a stand-up guy. I didn't read it, but I hope you don't have kids.
Are you asking if you are crazy or your 28 year old is? You don't seem to have much self-control, which is clearly why you are alone now. Doesn't seem much more to say. Any advice we give won't change a thing. You want what you want and to hell with anyone else. I'm not going to even try to appeal to your better nature, since I'm not convinced there is one. She's a cheater too, and people generally get who and what they deserve.
You may be 40, but Vash is right. You have the mentality of a teenager. Good luck.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh