The panic attack scene with Carmy and my understanding of why thoughts of Syd calmed him down is that she, unlike the people from his past (including Claire), is the one thing he feels he got right/is getting right in his life.
Before taking on the Beef and meeting Syd he was a walking shell, dead inside. A disappointment to himself and also his family (at least in his head). He didn't feel like he measured up. He didn't feel like he was enough and even less for others.
But in the present, which Syd signifies, he has yet to let people down (especially her) and he strives on this path of ensuring he never does. She looks up to him while the people from his past often had reason to look down on him or doubt him.
Long story short, I feel like Sydney is his relief. A breath of air he needs when he's ran out. The assurance he needs that he's getting something right.
It was why he lost it in s1 when she up and left. It's why the thought of her got him back together during the panic attack.
Corny, but it fits to me.