I wondered how Henleigh Grandcourt Esq. had managed to subdue Gwendolin so oppressively, considering she was so "saucy" and high-spirited before their wedding. Chapter 45 gives a clue:
Gwendolin's heart began to beat violently. The words that she wanted to utter, as one wants to return a blow, were, 'You are breaking your promise to me - the first promise you made me.' But she dared not utter them. She was as frightened at a quarrel as if she had foreseen that it would end with throttling fingers on her neck.
Poor Gwendolin, so Grandcourt roughs her up. That does make her life seem hellish, despite her wealth and privilege.