"Once this. Your long experience of her wisdom."
'Her' is Rowe's correction of your of the folio.
"And in despite of mirth mean to be merry."
To be merry in spite of mirth is like laughing in spite of laughter, dying in spite of death, living in spite of life—pure nonsense. With great confidence I therefore made the correction my wife, and so gave it in my Edition. Meeting, however, in the Cambridge Edition with Theobald's correction wrath—for Editors had ignored it—I saw at once that the poet must have written my wrath (see Introd. p. [67]), which resembles 'mirth' both in form and sound, and I have therefore adopted it without hesitation. Like a similar correction in Twelfth Night, i. 1, I regard it as absolutely certain.
Sc. 2.
"Shall love in building grow so ruinous?"
So Theobald, in accordance with the rime, read for ruinate of the folio.