i really know how you feel . i been having sex with my ex to because i don't have any friends and no family support ...even though my ex will treat me badly emotionally afterwards. all that we can do at this time is try to be strong and break this bad pattern and get our self esteem back so we no longer allow ourselves to be used. As for the other woman it sounds like your ex and her deserve each other , you deserve better but i know how much it hurts.
If that woman is abusing her children maybe you should talk to a lawyer to see if what you can do to keep the children from that sitution for thier best interest.