I don't think Peter would have actually left Carla for Tina, Tina was just too loved up to realise that she was abit of fun that got out of hand in Peter's fucked up little head he always loved Carla ( it makes no difference whatsoever though he still had an affair ) .
I know theres alot of hate for Peter, but underneath it all there is an awesome person. Just like with Carla. I get why they have done the things they have done