erik, i hate to say this, but the sooner you drop the "soulmate" routine and realize that she's got a lot of freakin problems, the better off you'll be.
my guess is you are going to have to eat a lot of crapola from her (this is a simple judgement based on the fact that any person able to fall in love with anyone else while married with a kid is pretty f#$%^d up, and exceedingly immature. AND divorce laws are extremely unfair and gender biased.)
i'm sorry to be so abrupt, but it's time to face up to things, considering there's a child involved. this is a HUGE thing in his life, so there's no time to be wishy washy.
i think andy gave you the best preliminary advice, from what i've read so far.
take care of yourself first. get used to catching yourself feeling sorry (for yourself) and like you can't understand what you did wrong. you didn't do anything wrong. it sounds like your only fault was choosing the wrong person to try to build a life. sh!t happens to the best of us. you will make a lot of mistakes in life, hopefully not all this big, but you're not perfect. accept it, and get ready to move on and not repeat them.
once you get your feet under you and forget the love-lost crap, only then can you be a competent parent. if you feel ok about yourself, you'll have no need to belittle your ex to your child, no matter what happens. in fact, your strength will help carry you and your boy through whatever the ex throws at you, and will solidify your father/son relationship. and if there is any possibility of reconcilliation - for the boy's sake if nothing else, then you can't be a patsy or doormat for a proven loser.
i haven't been divorced, but have been very close. fortunately, nothing has been done on either part to force a hand in my case. life is a work in progress, i hope.
i have been through 2 very rough breakups of long term relationships tho, losing an awful lot of dreams, possesions, and time along the way. dreams and possesions can be regained. the last cannot, so realize that every move from now on must be thought out, not based on emotions.
infidelity is about the worst thing to get over. been on both sides of that. no matter how much someone is sorry, the relationship is damaged, and at best it will take many many years to even begin to get back to square one.
well, whatever you decide, good luck. i hope it works out for you and your boy, and your ex realizes the amount of pain she's caused, and does the right thing by her son.