Stop sleeping with him.  You are divorced, he has stated and shown through his actions that he no longer cares for you or respects you.  And, make sure you have a legal visitation schedule, and document any time that he doesn't honor it.  Continue to pay your child support, and make a plan to challenge custody to at least get joint custody of your son.  No matter what, be civil to your ex no matter how ignorant he acts.  Don't let someone like that make you lose your peace, your joy, and your self-worth.  But most importantly, stop sleeping with him.

@NoSmiles The worst thing you can do in this situation is to demolish your self-esteem. I know things seem to be at an all time low but with time, things do get better. If you are still sleeping with your ex, you are still letting him control you emotionally. Stop. No matter how much it hurt, it is time to stop. I continued to sleep with my ex for a while too but my self esteem was sinking to an all time low. When I realized I needed to stop focusing on what happened in our marriage and move on then everything worked out. You have to put more focus on yourself and do things that make you feel better. If it mean talking to a therapist to get you over the rough patch in your life, do it.

I am sorry to hear about your situation!  But you have to accept your responability in the matter and move on.  What he has done in retaliation is not cool.  You may feel a bit responsable for this and you may be, but you have to let that go because it is self destructive.  You made a mistake.  How long do you want to pay for it?  You decide that!  Ar you so caught up in this man that you have to have him any way possible?  Is the guilt of knowing that you may be the reason for all of this weighing on you so much that you are offereing yourself to him as payment?  If so you need to re visit the situatiuon, come to terms with it, accept it, and then drop it!!!!  Move on.  everyone make mistakes and the thing that most people dont realize is that we pay for our mistakes as long as we want to!  we decide when enough is enough.  Some people feel a self rightouos need to suffer!  I dont!!!  Dont put yourself thorugh that situation.  Believe me it is you putting yourself through it, not him!!!  Your guilt is driving you towards this self destructive behavior!

@Nosmiles:  I can certainly see why you chose that screen name...your situation is definitely nothing to smile about.

I agree with the advice you've received from the FH Fam...STOP SLEEPING WITH THIS DUDE!  And stop beating yourself up about the situation. 

Perhaps you've convinced yourself that you're still in love with him, or maybe you're doing it to spite your sister (and her stank behavior is whole 'nother blog), or maybe its like Smoove said...its guilt.  Whatever is motivating you to continue dealing with this guy, you need to confront it, deal with it, and take the lessons & leave the baggage. 

In the words of Eleanor Roosevelt: "No one can make you feel inferior without your consent." 

@nosmiles - I agree with most of what has been said here. All I will add is that you are not alone. Plenty of women and men have hit rock bottom, been stuck on stupid and continued relations with someone they needed to cut off, myself included.  Get centered and focus on becoming healthy and whole.  Keep writing - even if not on this site, in a journal.  Get your feelings out on paper and through reflection/meditation. Don't seek out revenge, but seek forgiveness of your sister and ex husband, not because you want them to be your best friends again, but because you have to let go of the bitterness to get better.  You are not alone.