What can I do if my ex wife is filing for full custody?

If your ex wife files to modify the joint custody plan you would have to resist that in Court. The girl friend situation can result in different outcomes depending upon the Judge and the Judge's belief system. Some of the questions I would think about asking you if I were representing your ex wife is: Your child is entitled to a stable home situation and if your girl friend is so stable why don't you marry? Is there something about her that causes you not to want to marry her? Do you think it sets a good example for your child to seeing you live with someone you won't marry? Etc.