There are some concepts like self sabotage you may want to read about it, and get some context
There's also the idea that traditional relationships, and all relationships, friendships, whatever, have a cycle, that doesn't have to be predictable, but there is always going to be an easing from the "high", the intensity of ANYTHING pleasurable, especially something as intoxicating as love, lust, strong emotional connection.
adding on that, generally speaking, most people starts to mirror kind of a marriage like state after 6 months, approx, it's what we are taught socially. And that element of domesticity, toothbrushes and sleeping same bed together, a lot of convos start to revolve around "what do you want for dinner" (instead of flirting/whose house do we meet up at, coming over at midnight and having sex for hours) starts to feel a little more safe, as in this person isn't going to ghost you, but also starts to feel a little stalled? or a little smothering.
To me, you either accept that, and work at it, keep things exciting, traveling, push each other sexually, or leave it, but also realize it will not be THAT different with another person, relatively speaking. That cycle will reveal itself again.
or you kind of participate LESS in that dynamic but that's like breaking from traditional norms, which isn't easy or socially accepted at all.