OK let’s have a look at this. He has been modelled (which is more effective than saying it) that calling you a fucking bitch is acceptable and right. And it was used against you whenever your ex wanted to signal negative emotions. So your son, because he was taught to do it, called you a fucking bitch because he was feeling negative emotions.
A good parenting expert will pick apart the stages of this. First, antecedents. If he is tired, ill, hungry, upset or anything like that, no emotive conversations. Leave it to a neutral time.
Then, how you communicate. You were essentially saying, “see more of my parents, and you should feel guilty if you don’t”. You feel guilty and you were trying to make him feel guilty. Because that’s probably been an effective way to try to get your needs met in a house that was abusive. You can’t be assertive, you have to be passive aggressive. But it is the worst way to communicate with him and he will get avoidant of the feelings it causes in him. And again, what has he been trained to do when upset? Call you a fucking bitch.
And finding a new way. You need to work out how to ask for things, communicate things and listen to things without blame, guilt or me-railing. And he will have to learn that calling you names isn’t the only way to have his needs met.
He doesn’t know why you did that because for 13 years you accepted it. ‘Fucking bitch’ is just as much a normal phrase to him as ‘cup of tea’ is to a person in a healthy household.
The people telling you how dreadful it is and how you can’t put up with it don’t understand that to him, there’s nothing wrong with it because the two most important people in his life parented him that way. You have to kindly, empathetically, and calmly, spend the next 13 years attempting to retrain him.
You won’t be able to do it without support. Please please read, learn, take every parenting course you can, get therapy, everything.