Therese did not even see any special lights. "Who's Abby?" (7.55)
This sentence has a layered meaning. The mention of Abby's name clouds Therese's vision—she should be enjoying this special time with Carol, but she's immediately jealous that another woman has entered the picture.
Therese thought with sudden envy, she could not make Carol laugh like that, but Abby could. (7.157)
We think Therese is being insecure here since people comment multiple times that Therese makes Carol laugh. Or maybe they're being sarcastic, and Therese is terrible at making Carol laugh—we never actually see Carol laugh with Therese, so maybe Therese is right to be jealous of Abby.
How well they must know each other, Therese though, so well that nothing either of them said or did to the other could ever surprise, ever be misunderstood. (7.186)
Again, Therese is jealous of Abby, here because she and Carol have a history that Therese and Carol do not. However, Therese will learn that history isn't everything.
Perhaps [Abby] was in love with Carol, too. It put Therese on guard with her. It created a tacit rivalry that gave her a curious exhilaration, a sense of certain superiority over Abby—emotions that Therese had never known before, never dared to dream of, emotions consequently revolutionary in themselves. (10.24)
The jealousy drives a competition between Therese and Abby, in a friendly way, and it also encourages Therese. If Carol can be close with Abby, it gives Therese hope that she can grow close to Carol, too.
"I think I understand better now."
"Just—that you win."
"What," Abby echoed with her head up, looking up at the corner of a building, at the sky, and Therese suddenly felt furiously impatient. (10.95-10.99)
Therese stops being jealous of Abby, both because Abby says she "won," and because she realizes how insecure and immature Abby is. Therese has no reason to be jealous of her because she is no kind of competition.
"I don't think Abby likes me," Therese remarked. "I don't think she wants me to see you." (11.11)
This is partly true, but Therese, who's young, doesn't yet understand Abby's complicated emotions. As Carol's friend, she wants her to be happy—but Abby also wishes that she could be with Carol, so she's torn about Therese.
Richard nodded his long head, still smiling the downward, disgruntled smile. "And you were alone with him, just the two of you." (12.43)
Richard is jealous of Phil, another man, when he should be jealous of Carol. But Richard doesn't suspect that Therese is in such a relationship with Carol. He'll be jealous of her later, though, when he finds out.
She envied [Richard]. She envied him his faith there would always be a place, a home, a job, someone else for him. She envied him that attitude. She almost resented his having it. (12.143)
This attitude isn't just toward Richard; it's toward all heterosexual people, who enjoy a security and complacency that a homosexual will never have in 1950's America.
"What really makes me sore is that you act like I'm not worth anything, that I'm completely ineffectual. It isn't fair to me, Terry. I can't compete!" (13.48)
Richard only feels jealous of Carol when he feels emasculated by her.
For an instant there came the fantastic realization that Carol had devoted only a fraction of herself to her. […] Yet the fact remained, she had chosen her child. (21.42)
Is Therese jealous of Carol's child? Would that be an immature attitude to have, or would her feelings be valid?