The "sitting in a grave" part refers to Yeats using his personal suffering as artistic inspiration, something Dolores knows about very well. As she goes on in the previous line "And you writing down everything, But I know by now" seems to reflect an earlier time in her life when she might not have realized why he put himself through the misery and in her eyes closer to his own death, though is it clear to her now as an artist having experienced her own heartbreak and healing through artistic expression.
The "sitting in a grave" part refers to Yeats using his personal suffering as artistic inspiration, something Dolores knows about very well. As she goes on in the previous line "And you writing down everything, But I know by now" seems to reflect an earlier time in her life when she might not have realized why he put himself through the misery and in her eyes closer to his own death, though is it clear to her now as an artist having experienced her own heartbreak and healing through artistic expression.