How we cite our quotes:
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.