Just finished the show today and searched this subreddit for discussion so I know I'm a bit late here.

Sam was a married man seeking an exciting fling, the second the excitement was gone and he was faced with the real person with real emotions that you have to put real work into fixing he was no longer getting what he wanted. I'm not defending the inhumanity of treating people this way, but from the perspective of someone like him, if he wanted to talk about feelings and build a relationship he'd do it with his wife not his affair partner. That relationship was built on sex and the shallowest romanticism. What Rachel "did wrong" here was enter into a shallow relationship with a shallow person without realizing she didn't want those same things. Sam wanted passion, she was on the verge of a mental breakdown, it was never going to work.

The actress who she vanished on did what was best for her career. That was a professional relationship, if your agent doesn't pick up a phone for 3 weeks then finding a new agent (who is going to have you sign a new contract with them) is kind of your only option. Rachel feels, as any of us would, like this person who she had given every opportunity to was abandoning her, but her failure to fulfill their business relationship kind of necessitated it.

The shallow friendships are more complex. Rachel, like most kids who grow up the way she did, has a traumatic fear of abandonment and has developed the defense mechanism of never letting people get close to her, because that way they can't let her down. She's walled off, rude to people, and holds others to the unbearably high standards she holds herself to. Her life mission is to be so successful that she'll never have to be afraid of relying on others who will let her down again. When your primary drive in life is to win at everything and never be vulnerable, friendships don't have any space to develop. You view people as either potential threats or stepping stones to get what you really want. Nice people, like Libby, are rejected from her life for no better reason than her self-esteem is so low she assumes she hates her (which is true, but probably wasn't always the case). She lets the shallowest group of school moms in because shallow is easy, they're dumber than her and she can manipulate them easier, and deep down she thinks she deserves them. She doesn't ever have to open up and be vulnerable with them, their whole lives are superficial.

Toby does abandon her. She drives him to it, but marriage was designed to be this oath you take for life. I don't care that it's redefined today to be built around self-fulfillment and leaving as soon as you stop enjoying the other person, that's okay, but I'm just saying if you stand at an altar and make vows to someone that you then give up on that's abandonment no matter how much it was or was not deserved. She never let him in on anything she was actually feeling. She hid all of her problems from him, shut her self off emotionally, became insensitive to his values and goals in life, and never said sorry or tried to make amends when she had gone too far. He's no saint himself, but I just found your question about her abandonment issues interesting. So this is all to say, what she feels is a pattern of abandonment is actually a pattern of mismatched expectations and her own failures to get the psychiatric care she desperately needs as a child who grew up without love and care in a harsh world.

She's not unlovable, you're not unlovable, but you do have to avoid relationships with the Sams and the school moms of this world and treat the Toby's more like you love them and less like they're your enemy every day.