Perhaps not thinking clearly, still grieving the loss of her love despite outward attempts to remain indifferent and strong, this a turning point for Starlight. She saw the two ‘together’ as a betrayal by her ‘friend’ and for this injustice denounced the teachings of friendship she had learned, as well as her teacher. Yet, despite her struggle not to release her pent up hurt and fury on Twilight and Sunburst, Starlight said nothing. Instead, she began to become cold to Sunburst, rather than faking happiness. It didn’t take long before Starlight began denying Sunburst the chance to visit Gamma Ray. In his absence, Gamma Ray’s head was filled with slander towards the ways of friendship and trained in magical prowess, that one day she might exact the revenge her mother could not bring herself to.
Sunburst tried valiantly to see his daughter at first, and even Twilight and Starlight’s other ‘friends’ attempted to reason with her. However, Starlight would have none of it and instead became more and more reclusive, keeping Gamma Ray farther and farther from the public eye. After some time, the struggle broke Sunburst’s spirit to try, now sure that Starlight would never let him see Gamma Ray again, and he stopped trying, left with only the memories of the sweet little filly and the scars seeking to take their place.
I might write an actually story-dialog thingy for this, but currently it’s 2:30 am and I am tired and already wrote more than I intended. So instead enjoy some angst, snuggling, and a cute filly Gamma Ray before she became maniacal and bent on burning Equestria to the ground.
Headcanon Note: I imagine that unicorns with the more lionlike tails use them expressively. A high held indicates boldness, cockiness, or aggression, a tail even with the spine indicates slight nerves, while a low, loose or gently curved tail represent calmness/happiness.