How do I accept this?

ok here goes, I have been seeing my boyfriend on/off since last xmas, granted not a long time but it has been a roller coaster of a relationship and feelings are strong. He was living with someone when I originally met him (online) We didnt start to have a relationship until his ex and he split up. His ex randomly text him one day to say she was pregnant, then she text to say she miscarried. Nothing added up in all fairness having been in the situation myself a while ago I know the pain and suffering and she didnt seem concerned about going back to work the next day and all the rest. Later found out she had made the whole thing up so that my boyfriend would go back to her....which he did. They split up afterwards not due to the fact that he found out she was lying but he says its because he just didnt love her enough to stay any longer. I took him back. (yes already I know how pathetic I sound!) anyways.....she is now pregnant ...again. This time its real. After many a sleepless night and more than a few tears I decided to stick by my man and support him. Now it turns out that the baby has only a 50% chance of being his due to her inability to keep her knickers on. She is currently living with the other person involved but she insists that this baby is my boyfriends. He has agreed to take a DNA test once the baby is born (Prob courtesy of Jeremy Kyle...yeh I know I sound like a cow) I suggested he needs to talk to her and be an active part of this childs life if the baby is his but to be honest the whole thing is eating me up. I said I would stand by him even being civil to her and offering her advice as she is very young and I wasnt trying to patronise her in the least but she is having none of it to the point of screaming I cannot have anything to do with this child once its born even tho I am willing to go through it with him long haul. My friends say cut your strings but my heart just cant.....how do I deal with this emotionally cos I want whats best but I cant help feeling selfish and wanting it to go away.