"[…] there is a period when all reasoning must yield to nature; that is past: and another, when excessive indulgence, having sunk into habit, weighs down the elasticity of the spirits so as to render conquest nearly impossible […]" (1.2.5)
St. Aubert's got the lowdown on how to persevere through grief. We think it's pretty likely that he's lecturing to Em just to get himself through this major mourning period.
"Farewell, my love!" said he, in a voice of solemn tenderness—"trust me we shall meet again—meet for each other—meet to part no more!" (1.13.72)
Valancourt is in it to win it. This is basically his promise to Em that he'll wait for her for as long as it takes.
To the hours, the months, she was to pass under the dominion of Montoni, she looked with apprehension; while those which were departed she remembered with regret and sorrow. (2.1.22)
Not to undervalue Em's emotions, but she definitely has a flair for the dramatic. At this point, how does she know that she'll suffer under Montoni's "dominion"?
"O could I know," said she to herself, "what passes in that mind; could I know the thoughts, that are known there, I should no longer be condemned to this torturing suspense!" (2.6.4)
Just hanging out at Udolpho isn't the bad part for Emily. No, it's not knowing what Montoni has done in the past. She can persevere just fine if she knows what's going on in his mind.
She had scarcely strength to remove from the room, and regain her own; and, when arrived there, wanted courage to remain alone. (2.6.46)
It's all this waiting around, then fainting, then waiting around again that's really doing a number on Em's spirits. This is her lowest point in the castle.
"O! Whatever are my own sufferings, may my heart never, never be hardened against those of others!" (2.11.37)
Who's got time to suffer and be Mother Theresa at the same time? Emily, apparently. It's really important to her that she doesn't lose sight of the little people.
When they reached the court, the monk gave her his blessing, and, after a lingering look of pity, turned away to the portal […] (3.5.6)
We don't see too many outsiders looking into Udolpho, but this is one such instance. The monk can clearly tell that that Emily's under duress.
Unwilling to encounter the coarse behavior of the peasant's wife, she remained supperless in her room, while she wept again over her forlorn and perilous situation […] (3.7.11)
Shmoopers, the girl's not in Antarctica. She's in Tuscany, land of sun and fun! Has she never seen Under the Tuscan Sun?
"I am myself a prisoner, a sufferer, like you." (3.9.17)
Du Pont is trying to endear himself to Emily by finding common ground. Dude, you put yourself in the dungeon just to be near Em.
"Valancourt! I was, till this moment, ignorant of all circumstances you have mentioned […]" (4.18.23)
At last, Em's perseverance has paid off. She's willing to marry Valancourt on her own terms, and absolutely not before his name is crystal clear. It's a sacrifice that gives her some major leverage in their relationship.