Absent Mommy

I have strong emotions about this to the point where I've told my SO hes afraid of his kids mom. brief recap he lost his kids to his wife due to alcohol addiction but he was young then hes 50 now..and thos new baby mom is awful. she only comes at the end of month to visit her son..barely calls.he says he cant make her..the son is Told to tell mommy he loves her. I know I shouldn't be THIS upset but since Im TTC and im with his son more than mommy ..I feel like im taken for granted. . Taking child to school..with him when sick..day night and mommy wants to be mommy on her terms. my SO sees it but hes like I cant make her and thats true. Just aggravates me to want a child and see a mother get called mommy and sees him once a month for $$$ oh well out of my control right????