>>127999120The way the show is constructed is that they usually foreshadow things and then explore them in the subsequent season. It was implied that Mariner has abandonment issues back in S1, but it was never explicitly stated. S2 spelled these things out for the audience and then introduced some more ideas that will likely be expanded upon in S3, and they will likely involve her love life or lack thereof.
Mariner is mentally a teenager.
It's been implied that she goes on a lot of dates but it ultimately amounts to nothing. She greatly exaggerates facts about her dating life to impress people. She's utterly incapable of communicating her feelings in a healthy way. Impressing her is often enough for her to fall for someone. She burns through short-lived relationships that are based solely on intense infatuation like a teenage girl.
Jennifer is a serious, career-driven woman. She might notice Mariner's lack of maturity, both sexual and psychological, and she might ultimately decide to break things off when she realizes that she's dating an immature girl who's in need of a therapist, not a girlfriend. I'm also inclined to believe that Jennifer is actually a rebound, but we'll see about that.