I've heard this
before from my colleagues, sad story but it's all up to your
husband. And the girl, she is still young and I guess she's single,
and from my view young girls only want to have fun wif older
married men because married men understand young girls better and
they know what young girls want in life, eg. care, concern,
communication, companion, abit of control, compromise, attention,
adventure, etc. In addition some married men are much more secured
than a single guy, they've got all the 5Cs, so the chances of
having another affair is there as you can see, especially those
married men who never stop exploring things in their career and
life 
I can only suggest to
you to talk to him or
her, ask him what he really needs from you, try to
understand and compromise, I guess from all the things he's done is
to get some serious attention from you, may be you're busy wif your
kids all this while and you didn't even notice that your
husband needs attention from you too. Honesty is the best
policy when talking to each other to solve this problem, it's ok to
do it now because it's not too late you see...... There is a reason why
he's doing this... and it all comes from you, ask yourself... and
think about it again...and ask him and solve things out.
Think about your kids .. it's not easy ... 
Wish you good luck
