great post crak600, this is real shit. Not great job for banging up a storm, just great post for being honest.
I don't think you are an asshole for cheating. I don't think it is right, but its tough to blame someone. You have been in a relationship a long time, its routine and probably mundane. You love your GF, and your kids, and your life, but at the same time, you crave attention from other women, new pussy excites you. newsflash, you are human.
Why do you think prostitutes are so prevalent in conference cities like Chicago and Orlando? Its married men banging them, because single guys can get it for free. Why do you think older married couples choose the life of swining, they want some legit way to get new pussy or dick, its just in our nature.
What do I think you should do? Figure out now what you really want. You've had a taste of some new pussy, do you want to continue doing that for the next year, 5 years? 10 years? your entire life? You've had a taste of domestic life with a steady loving GF and kids, do you want that? Is that how you see your future? First figure your shit out and be true to yourself.
Then as far as telling the GF. I'd use situation ethics I suppose, you are human, you make mistakes. You may be able to stop yourself, but if she knows, she might not be able to forgive you, or always have doubts. Its a tough call, if you tell her, you have no guarentee of anything.
I don't think things would last with the current young chick so just forget about that right now. You want her for the wrong reasons. When you are 50 you aren't gonna care that she can fuck all night and she certainly won't look as tight as she does now.
My advice, get counseling, let this out to someone professional. Talk to superdave about sex addiction as well. Who knows, I am not sure I believe in that, but it may be a starting point.