It's just something you are going to have to deal with. It won't be easy at first, but in time it will be. He has the right to move on with his life and include his daughter within that life. The best advice I can give you is not to despise his new girlfriend. You need to be able to get along with her from the get go. Your daughter will not deserve the immature bickering and arguements. Be an adult about this, and set the example for your daughter. Best wishes!