Quote 1
"Elinor," cried Marianne, "is this fair? is this just? are my ideas so scanty? But I see what you mean. I have been too much at my ease, too happy, too frank. I have erred against every common-place notion of decorum! I have been open and sincere where I ought to have been reserved, spiritless, dull, and deceitful. Had I talked only of the weather and the roads, and had I spoken only once in ten minutes, this reproach would have been spared." (10.5)
Again, we see Marianne struggling with notions of what a proper lady does. While she can recognize crimes against convention in others (such as Mrs. Jennings), she always hotly defends her own conduct when it's outside the realm of the ordinary or proper.
Quote 2
"I have not a doubt of it," said Marianne; "and I have nothing to regret -- nothing but my own folly." (47.8)
Marianne's views on love are clearly changing – she recognizes that she herself was a fool for love with Willoughby, and now repents.
Quote 3
Many were the tears shed by them in their last adieus to a place so much beloved. "Dear, dear Norland!" said Marianne, as she wandered alone before the house, on the last evening of their being there; "when shall I cease to regret you? -- when learn to feel a home elsewhere? -- Oh happy house! could you know what I suffer in now viewing you from this spot, from whence perhaps I may view you no more! -- And you, ye well-known trees! -- but you will continue the same. -- No leaf will decay because we are removed, nor any branch become motionless although we can observe you no longer! -- No; you will continue the same; unconscious of the pleasure or the regret you occasion, and insensible of any change in those who walk under your shade! -- But who will remain to enjoy you?" (5.8)
Marianne's lament to Norland is a heartfelt one – she sounds melodramatic, but she's serious. Imagine leaving the only home you've ever known – that's about how the Dashwoods feel! Underneath Marianne's poetic pretension, there's genuine feeling.