Why did they make the mother seem so heartless at first

Because the first two we saw her, was from her son's perspective. She even admits herself that she wasn't a good mother to them, and that's what he sees: "a mother who wasn't there for me".
Whereas now, we see things from her perspective: "a mother who wasn't there, but now regrets it"

*the first time we saw her

Ur explanation is gold