This might sound kind of crazy because it's not the norm, but I'd contact the girlfriend and be up front, but nice. Tell her you're sorry (if you are) about having had an \"affair\" with him since you knew nothing of her at the time. Tell her you don't want him for yourself, but you'd like him to be a dad to your baby, both financially and emotionally, and you need her help. After all, her baby and yours are half siblings. Tell her for the benefit of both of your children that you'd like to form an alliance with her, if not a friendship. But, who knows, you may end up becoming great friends. You can support each other through all the things that come with motherhood: babysitting, illness, milestones, etc. I think if you approach her with the right attitude and keep your anger with him in check, it could be really good for all involved. Remind her that despite her hurt and anger towards you, your baby and hers don't deserve to be punished by not knowing each other (and yours being supported by a father). Even if it doesn't work, it can't be said that you didn't try, and give her time to warm up to the idea. It may take some time for her to give it a shot initially. Good luck to you. I hope it works out for all involved.
