There are very clear character arcs for all of the main characters inhabiting world of New Girl. Apartment 4D is a place of growth and development. Over the years, the characters start to take relationships and careers seriously, build families, and slowly drop their immature behavior.
When Jess moves into the loft, she's lost. A tough breakup sees her struggling to move on. Though she's a teacher and exhibits a lot of mature personality traits, she's also got a lot of immature ideas she has to work on. As much as she grows up over the course of the series, she often takes a step back, backsliding as Schmidt would call it, regressing to behaviors the audience thinks she should move beyond.
Grows: She Goes After Her Dream Job
Jessica Day is one of the hardest working teachers - even if it's not always clear what subject she actually teaches (she describes herself as an English teacher early in the series, but is clearly teaching middle school science after that). When Jess loses her job, she flounders, and the audience can't help but empathize with her.
It takes Jess a long time to gain her confidence back and really go after what she wants. When she does get a new teaching job, she's frequently taken advantage of by other teachers and the administration. It's not until Jess decides to really go after the job she wants - being vice principal - that she starts to grow a backbone in her career.
Regresses: She Institutes Middle School Dance Rules For Nick
Before Jess and Nick become a couple, they struggle to deal with their feelings for one another. It seems like Jess is the mature one. She wants Nick to talk about things with her, but she also is in extreme denial about what's happening between them.
Instead of dealing with the situation like adults, Jess informs Nick that they're going to observe "middle school dance rules" to limit physical interactions. It's not clear why she thinks that's going to work since they tend to jump into one another's arms every time they're alone. The "rules" only last a couple of episodes and serve to make their interactions even more awkward.
Grows: She Gives Sam The Letter
There is no world that exists in which Jessica Day doesn't tell someone that the person she thinks they should be with is in love with them. She's not great at keeping Cece and Schmidt's feelings for one another a secret, after all. When she tries dating Sam (again), she finds out his best friend from medical school still harbors feelings for him, and has even written him a letter.
The old Jess would probably have taken her time to process the information, even wait until the other woman is out of the city before she makes a decision. Instead, the more mature Jess takes Sam to the other woman's hotel and gives him the letter, encouraging him to figure things out.
Regresses: She Plies Her Coworkers With Alcohol
A lot of Jess' moments of regression in the third season are a result of her relationship with Nick. Though both of them do a lot of growing early in the show, they do a lot of immature things once they're together.
One of those is Nick's idea of getting the other teachers at her new job to like her. Instead of being herself and accepting that not everyone clicks with her - something she's actively worked on - Jess decides to listen to Nick and bring this trio of teachers to the bar. He gives all of them drinks into the early hours of the morning, and then Jess is almost too hungover to go into work.
Grows: She Confronts Nick About Her Place In His Relationship With Reagan
In the fifth season, Jess leaves the loft, seeing herself sequestered during jury duty. While she's gone, Nick starts a relationship with Reagan, and though Regan and Jess get along well when Jess returns, something is still off.
Jess often finds herself in the "girlfriend" role in Nick's life when Reagan is out of town for work. The old Jess would have accepted that the things she does are part of her friendship with Nick, but the Jess who knows she put Nick in a similar position in the past decides to confront him. They actually have an open and honest conversation about how uncomfortable she is, and Nick makes an effort to treat her differently.
Regresses: She Attempts To Trap Her Parents
In the early seasons of the show, Jess finds comfort in her favorite movies. She also takes inspiration from one of her favorite childhood movies as an adult.
As a kid, Jess used to force her separated (and later divorced) parents into situations together, hoping to "parent trap" them into getting back together. At 30, Jess decides to give it another go. She gives them both the same time for Thanksgiving dinner in one of the many holiday episodes of the series. Jess instructs Nick to flirt with her mom to make her dad jealous, and she gets her hopes up when she spots her parents in a compromising position. It's one thing to wish your parents could get along better; it's quite another to try to trick them into a relationship.
Grows: She Watches Less Dirty Dancing
The love Jess has for Dirty Dancing has no bounds. It's her go-to comfort movie after suffering through a bad breakup. In the pilot episode, she wallows in her grief and watches the movie for days on end.
While it's perfectly acceptable to have a good cry - and everyone grieves in their own way - it's clear that Jess moves beyond watching her favorite movie to get through a tough time. In the third season, following her breakup with Nick, she again watches Dirty Dancing, but she does it while talking with him and accepting his comfort in their weird situation. Her breakups after Nick don't feature Dirty Dancing at all. Everyone deserves escapism, but Jess learns to deal with breakups.
Regresses: She Runs Away To Portland
There are a lot of big talks from Jess about dealing with feelings, facing fears, and confronting situations. She helps her roommates with them often. When push comes to shove, however, Jess decides it's better to escape her living situation instead of figure things out.
In season six, Jess admits to herself (and Cece and Schmidt) that she's still in love with Nick. It makes things incredibly awkward for her since she genuinely likes Reagan and is still so close to Nick. Instead of laying everything out on the table - or paying attention to Nick and Reagan's crumbling relationship - she decides to run. Specifically, Jess runs to Portland to stay with her dad and dodges Nick's calls when he's worried about her.
Grows: She Stops Trying To Change Her Roommates
Season one's Jess wants things a certain way. That can cause a lot of conflict for her when her roommates already have their own system in place. Jess spends a lot of her energy trying to get the guys to behave in a way she wants them to. Whether that's getting Schmidt to loosen up or getting Coach to spend more time with her, Jess uses manipulation to get her way.
The older she gets, however, Jess accepts that she and her roommates are very different people. Over time, she lessens her tendency to control situations, even agreeing to do things their way instead.
Regresses: She Returns To Her Exes
Schmidt, Winston, and Coach make a big deal about Nick going back to Caroline in the first season of the series, but no one is more guilty of backsliding in relationships than Jess.
When she experiences bad breakups, or when she finds herself lonely and missing what she used to have, she goes back to her exes. Jess almost gets sucked back into Spencer's drama in the second episode of the series! She also revisits her relationship with Paul, almost costing him his new girlfriend. Jess breaks up and makes up with Sam multiple times, even though they always agree they aren't a good fit.