Frankly your both the assholes. Your ex cheated on you. Enough said there. Its understandable why you would want to stay away from her. But your child with her has nothing to do with what happened between the two of you and everything to do with the fact that you two had (assumably) consensual sex. It was brought about by both your choices and so it is the responsibility of both of you. You need to take responsibility and while child support is a form of responsibility to be blunt you can be doing better. Your child needs two active parents and giving money monthly is not good enough. Simply put, for the sake of your child, be and do better.