Hi mum to matt
This is my first post, after reading your post, I HAD to reply. First let me tell you I feel so bad for you. Personally I wouldn't stay with him, but no one can make that decision for you. If you do stay with him, make sure you aren't just doing it so that he can't "play happy families" with your baby and his girlfriend. I know this would eat me up inside too, but it's no reason to stay with someone. His girlfriend may not want him anyway, I wouldn't if my partner fathered a baby with someone else. Also you need to think about the example you are setting for your children if you stay with this man, your son will think it is ok to treat a woman like this and your daughter will think that being treated second best is ok. And it's not, you deserve so much better and so do your kids. This guy doesn't deserve you, and YOU deserve so much better, there are plenty of fabulous guys out there who will treat you like a queen and the more time you waste with this guy, the less time you could have with someone wonderful. All this said, sometimes you have to be with someone to realise you can't be with them. Even if you did forgive him and stay together, could you do that and not throw it back in his face all of the time, and not be eaten up with insecurity and mistrust? Because that wouldn't be fair on anyone involved.
Good luck with whatever decision you make, this would be hard enough to deal with not pregnant, let alone pregnant. Don't forget that you will get through this and life does go on, just trust yourself to do what's best for you and your children.