Cracks in My Heart
The Doctor did something she hated. She turned away from someone in danger. It does against everything she stands for and her heart aches every time she is forced to do it. But she had to. If she didn't, Yaz wouldn't exist.
She remembered she felt the same way watching Rosa Parks being taken away, her head held high. It cut deep inside her when she couldn't save someone, or someone died because of her.
She tried not to listen to the gunshot, but it pierces through the silence and she closes her eyes briefly. Out of the corner of her eye, she sees a tear spilling down Yaz's face. The Doctor reaches out and takes her hand, as if to reassure her, but also to say sorry.
The Doctor is sorry they had to leave Prem behind. She is sorry they had to walk away and not even face what was happening. She is sorry that Yaz had to be here, to see this, witness it. She wasn't meant to. But what's done is done, and the doctor can't change that, or save anyone, even is she feels she needs to. It is her natural instinct to save everyone and anyone she can, even if it is putting her own life at risk. It was simply her nature. She couldn't change that, so, she hated it when she was forced to go against it. It's like her heart forms a new crack every time it happens. The cracks don't ever fade, and she remembers each and every person she has ever left behind.
But she has saved far many more, she just needs to remember that sometimes…