This means that after Baoyu married Baochai, he still can't forget his sister Daiyu.
What I find difficult to express here is the regret and sorrow for people or things that have been lost and can no longer be owned. Or maybe the feelings or things that should have belonged to them have taken an unexpected turn, and it is difficult to balance in my heart for a while after losing them, leaving me with remorse, regret and indignation.
generally speaking, you are suffering from some sadness that should not have happened to you.