You're The Worst has always been Gretchen and Jimmy's story, but that story will soon come to an end as the show's fifth and final season winds down. However, it's starting to look like despite all the hardships they've worked through, Gretchen and Jimmy may not be endgame after all. You're The Worst seems to be leading to their marriage and a happily-ever-after, but to paraphrase another show in which things tend to go pretty poorly for protagonists, "If you think this is going to have a happy ending, you haven't been paying attention."
In the Feb. 13 episode, Jimmy and Gretchen decide to take a brief break in order to have what Jimmy gleefully refers to as a "f*ck week." Gretchen readily accepts the offer, unaware that the reason Jimmy is suggesting the week of no-strings-attached sex with other people is because he just cheated on her with their wedding florist. While Gretchen is excited at the prospect, she soon realizes that her lust for meaningless sex has been replaced by the sense of meaning and purpose she finds in work and in her relationship with Jimmy.
However, after putting it off, she finally goes out and has sex with a "rando" on the final day of f*ck week so that Jimmy won't feel bad for having sex with other people. That is, until she finds out that Jimmy never had sex with anyone during fc*k week and only suggested it to make him feel less guilty about his infidelity.
Basically, Jimmy conned Gretchen into having sex with other people, which she wasn't interested in doing, so that he didn't have to feel bad about having betrayed her trust. Jimmy does some growing of his own, but his version of growth is learning that it makes him sad when his fianceé has sex with other people. Gretchen seems to be developing and growing as a person at a much faster rate than Jimmy, and if Jimmy can't catch up and grow alongside her, it could lead to these two permanently parting ways.
You're The Worst has always been a satire about how people rely on love to serve as a bandage for all of their own problems, but Gretchen seems to have finally figured out that a good relationship cannot make up for an unhappy life. She needs to focus on making herself happy, first, and her relationships will follow. Jimmy, on the other hand, is too busy trying to ensure that Gretchen is on the same moral level as him, dragging her down whenever he messes up. Gretchen is rightfully furious when she discovers why Jimmy manipulated her, which is only the latest in a long laundry list on ways these two have hurt each other.
While the two have put five seasons worth of work into their relationship, it seems as though it'll only be a matter of time before they realize that the sunk-cost fallacy is just that: a fallacy. So long as Jimmy is still, well, The Worst, and Gretchen is steadily becoming better, their years of progress may not be enough to keep them together in the end; eventually, Gretchen will outgrow him.