Hi Nino, it’s one of my favorite anime series haha. I actually thought Kousei’s letting go of his past is quite well developed. It’s very subtle, though.
His mother was the sole reason he plays. So when he lost her, he lost his reason to play. But it was through Kaori that Kousei found another reason to play. And when he decides to let Kaori enter his life, he ‘replaces’ his past. That’s the way I interpret the series: Kaori presence made it possible for Kousei to let go of his past. His forgiving his mother wasn’t explicit at all.
But the heartbreaking/optimistic thing is that Kousei had to learn that his reason to live and play shouldn’t be dependent on anyone at all.