Deep down I know its my mums fault that my parents are getting a divorce, but why do i want to take the blame?

It is NOT your fault that they are getting a divorce. They were already going to get one before you found the texts on her phone. Your Dad already had suspicions. You did the right things by showing your Dad the texts. You may be feeling that it is your fault as you showed the texts to your Dad and you could see that as the ultimate thing that led to divorce. It is not and you should not blame yourself at all. You have been very brave by showing your Dad the texts and have done the right thing. Good luck