Cheating ex

Has anyone ever been left for another woman, not just any woman but the woman your ex boyfriend was cheating on you with while you were together and pregnant? Now he acts as though you or his other kids doesn’t exist and has had a daughter with the other girl? Every time I call because his kids wants to talk to him and see him e avoids my calls.. when he does chooses to answer he promises my kids that he will be by to see them but never shows.. I reached out to his girlfriend on Facebook wanting to establish a relationship for the kids sake but it ended bad because she thought I was being smart… she later on wrote me back and apologized but I ignored it and just blocked her.. idk what to do?!! Has anyone gone though something similar, what should I do?