Daria's life has gotten more complex. When she was a Total Outsider, or could reasonably pretend to be, it was easy to just do what she wanted and damn the consequences, because there _were_ no real consequences.
But now she has real friends. She has a boyfriend. She's soon going to have to make deecisions about college and a career.
Suddenly, she's making decisions about more consequential things than what sort of cutting remark would be most appropriate. She can hurt or alienate her friends (as she did Jane in _DDMD_). She could hurt and lose her boyfriend (as she nearly did in _IIFY_).
Should she have sex with Tom? If she does, then this makes it (in her mind) something which will either be permanent, or a mistake which will humiliate her for a long time to come. (*) If she doesn't, then is she really loving him? This is a difficult decision -- Tom's making it easier by not pressuring her, but it's still not easy.
What college should she go to? What should she major in? What career should she aim for? Whatever choice she makes, it will close off other possiblities.
To take one obvious example, should she pick a college that's cheap? That has a good program in her majors? That she can go to with Tom? With Jane? What does she do if there's no one college that fits all these criteria?
During most of the series, Daria's only responsibilities were: 1) Get good grades, and 2) Stay out of major trouble (sex, drugs, violence). If she did these things, her parents would give her a food and pay for most of her expenses, and that's _all_ she had to really do. And, given Daria's intelligence and morals, these were _easy_ things for her.
Life's getting more difficult now.
Welcome to adulthood, Daria.
(*) Helen meant well, but she was _not_ helping by talking about the stunt car driver.