Nakamura didn't forgot about Kasuga, they did share a bond but they had a toxic relationship, she pushed him away from double suicide so he can move on but honestly, after trying to commit suicide and all that shit it would be fucking hard to live on, I know firsthand how one person choose to end herself.
Nakamura did the same thing when Kasuga was going out with Saeki after she found out, pretending to not care, that was heartbreaking.
Almost gave up on Kasuga when he ran and grab Nakamura, like he almost gave up on Tokiwa, but as they fought and Tokiwa joined in that felt wholesome.