More Information ...

Ancient damnation: damnable (wicked) old woman
bosom: secret thoughts
twain: separated

Juliet is very upset that the Nurse has disparaged Romeo. She can't decide which is worse: the fact that the Nurse has decided Juliet would be better off married to Paris ("wish me thus forsworn") or the fact that the Nurse has said nasty things about Romeo. It hardly matters. Juliet chooses to sever the relationship with her Nurse. She will not confide in her anymore.