divorce can be very hard on the parents. i would say give her time to heal, i dont know if a year is enough..
There is absolutely nothing wrong with talking to your dad, and your mum shouldnt expect you to stop talking to him.
just try not to get angry with her cos shes only feeling hurt with you talking, if you get angry it'll make it a whole lot worse, have you tried to comfort her, talk about other things? get closer in general, build a good relationship and then she'll understand that you need your dad.