For a long while she had been oppressed by the indefiniteness which hung in her mind, like a thick summer haze, over all her desire to make her life greatly effective. What could she do, what ought she to do? – she, hardly more than a budding woman, but yet with an active conscience and a great mental need, not to be satisfied by a girlish instruction comparable to the nibblings and judgments of a discursive mouse. (1.3.13)
Dorothea has all kinds of energy, but no obvious way of exerting herself. Her friends and family want her to be "satisfied by a girlish instruction" – in other words, to be content with the kind of education and pastimes considered appropriate for girls during the period. She doesn't feel like that's enough, but she doesn't know what else to do. And that uncertainty is "oppress[ive]" to her.
a social life which seemed nothing but a labyrinth of petty courses, a walled-in maze of small paths that led no whither, the outcome was sure to strike others as at once exaggeration and inconsistency. (1.3.13)
Dorothea feels that the "social life" laid out for her by conventional society is pointless – it's a "labyrinth." Any path she chooses to take is just one of many "petty courses" that leads nowhere. The fact that labyrinth is "walled-in" is important, too – she feels trapped by the expectations of society.
Mr Casaubon apparently did not care about building cottages, and diverted the talk to the extremely narrow accommodation which was to be had in the dwellings of the ancient Egyptians, as if to check a too high standard. (1.3.42)
One of Dorothea's passions has to do with making good cottages for the poor people in Middlemarch. She tries to get Mr. Casaubon to discuss her ideas, but he's not interested. He's more interested in the "ancient Egyptians" than in what's going on with the poor folks in his neighborhood in the present.
We mortals, men and women, devour many a disappointment between breakfast and dinner-time; keep back the tears and look a little pale about the lips, and in answer to inquiries say, "Oh, nothing!" Pride helps us; and pride is not a bad thing when it only urges us to hide our own hurts – not to hurt others. (1.6.78)
Here's yet another passage in which the narrator uses the first person plural ("we") to incorporate the reader, the narrator, and all the characters in one universal statement. She says that everyone is disappointed by something everyday, but that we all hide it to keep up a strong front. The narrator makes disappointment and dissatisfaction into a universal human condition, rather than something that afflicts a few people every now and then.
[…] the Vicar felt himself not altogether in the right vocation. (2.17.27)
Mr. Farebrother's dissatisfaction with his life stems from a mistake he made a long time ago, well before the start of the novel. He's in the wrong profession, and there's no good way to switch professions in late middle age. Nowadays, if you don't like your job, you can always go back to school and start again (it might be difficult, but it's generally possible). But Mr. Farebrother is trapped by that early decision. He's a good person, but he wasn't meant to be a clergyman. He should have been a biologist, because that's where his interests and passions really lie.
To those who have looked at Rome with the quickening power of a knowledge which breathes a growing soul into all historic shapes, and traces out the suppressed transitions which unite all contrasts, Rome may still be the spiritual centre and interpreter of the world. But let them conceive one more historical contrast: the gigantic broken revelations of that Imperial and Papal city thrust abruptly on the notions of a girl who had been brought up in English and Swiss Puritanism, fed on meager Protestant histories and on art chiefly of the hand-screen sort; a girl whose ardent nature turned all her small allowance of knowledge into principles, fusing her actions into their mould, and whose quick emotions gave the most abstract things the quality of a pleasure or a pain; a girl who had lately become a wife, and from the enthusiastic acceptance of untried duty found herself plunged in tumultuous preoccupation with her personal lot. (2.20.5)
Dorothea isn't able to comprehend the splendors of Rome because of the limitations of her "Protestant" education. All of the art and history in Rome seems to be just beyond her ability to grasp. Besides, she's "preoccup[ied] with her own "personal lot." So Rome, the narrator suggests, is an acquired taste – you have to go in already knowing something about its history in order to appreciate it. Dorothea's dissatisfaction while she's in Rome stems from her inability to get her head around it all – it's just too overwhelming.
Dorothea had no such defence against deep impressions. Ruins and basilicas, palaces and colossi, set in the midst of a sordid present, where all that was living and warm-blooded seemed sunk in the deep degeneracy of a superstition divorced from reverence; […] all this vast wreck of ambitious ideals, sensuous and spiritual, mixed confusedly with the signs of breathing forgetfulness and degradation, at first jarred her as with an electric shock, and then urged themselves on her with that ache belonging to a glut of confused ideas which check the flow of emotion. (2.20.5)
Dorothea has a hard time seeing the forest for the trees. She can't understand Rome's entire history because she just sees all the component parts – "basilicas, palaces, and colossi" and the "sordid present" of dirty streets and soot from coal-burning factories. She can't synthesize what she sees into a harmonious whole, so everything is a "confused" mix that she finds "jarr[ing]."
[…] the gentlewoman's oppressive liberty. (3.28.4)
Part of Dorothea's sense of dissatisfaction comes from her position as an upper-class woman: she has a ton of passionate energy, but nothing to do. Her servants do everything for her. Even though she's technically at "liberty" to do what she likes with her time, she finds the liberty "oppressive" because of the limits placed on it by social expectations.
Her blooming full-pulsed youth stood there in a moral imprisonment which made itself one with the chill, colourless, narrowed landscape, with the shrunken furniture, the never-read books, and the ghostly stag in a pale fantastic world that seemed to be vanishing from the daylight. (3.28.4)
Dorothea's sense of "oppression" gets worse. She feels "imprison[ed]" by her life. Even the "furniture" seems "shrunken" and the "landscape" is "narrowed." Her interior state makes her project her feelings on the exterior world. This is what a Victorian writer named John Ruskin called the "pathetic fallacy" – the assumption that your own interior state of mind could be projected on the world around you.
It is an uneasy lot at best, to be what we call highly taught and yet not to enjoy: to be present at this great spectacle of life and never to be liberated from a small hungry shivering self – never to be fully possessed by the glory we behold. (3.29.3)
Casaubon's dissatisfaction comes from his inability to see anything greater than himself. He's so hung up on his "small hungry shivering self" that he can't see the "glory" in the world or the "great spectacle of life." But Eliot, once again, switches to the first person plural ("we") in this description. She implies that this is something that "we" all experience. She takes Mr. Casaubon's individual, particular experience, and makes it into a general and universal statement about human nature.
Becoming a dean or even a bishop would make little difference, I fear, to Mr. Casaubon's uneasiness. Doubtless some ancient Greek has observed that behind the big mask and the speaking-trumpet, there must always be our poor little eyes peeping as usual and our timorous lips more or less under anxious control. (3.29.3)
Mr. Casaubon's dissatisfaction wouldn't have been made any better by an exterior change, like becoming "a dean or even a bishop." Those changes might have made his position better in the eyes of the world, but they would have been like a "big mask" that he was hiding behind.