Frodo. Poor dude isn't doing so well.
It seems like every single shot of him is a close up on his face, emphasizing the distant look in his eyes. He's not feeling too hot, either emotionally or health-wise. It's like the Ring is physically heavy, not just dragging on his mind, but also taking a toll on his frail body. Even Frodo's massive feet aren't big enough to carry the weight of the One Ring.
He's also more and more removed from Sam. Although they shared their troubles in Rivendell (and even as they started their journey apart from the group), Frodo is now emotionally isolated from his bestie.
It's partially that Frodo's unwilling to burden Sam with worries of the Ring, but he also believes that Sam can't understand the struggles of the Ring bearer. As Galadriel tells him in Lothlorien, "to bear a Ring of power is to be alone."
The happy Frodo we see in the Shire tavern at the beginning of The Fellowship of the Ring is long gone. Instead, he is constantly zoning out, staring into the eyes of the dead or the great eye of fire. His spill in the Dead Marshes shows how dangerous his disconnection with the physical world can be. He seems almost drawn to the ghostly spirits of the corpses in the swamp and comes pretty close to lighting a little candle of his own, as Gollum would say.
The New Gollum
All of these signs point to him becoming more like the nine wraiths, each of them undone by their greed for power. And he's also becoming more like Gollum. Remember the scene where Frodo is up at night, softly stroking the Ring in the palm of his hand? Right next to him we see Gollum, pretending or hallucinating that he has the Ring in his palm, doing the exact same petting motion.
We know it took five hundred years for Gollum to end up like he is, but with Sauron's army growing and the Ring getting closer to home, Frodo is this close to stripping off his clothes and biting into some raw fish.
Could this be why Frodo has so much pity for Sméagol? Check out Frodo's pro-Gollum stance in action:
SAM: He's up to something.
FRODO: Are you saying there's another way into Mordor?
GOLLUM: Yes. There's a path, and some stairs, and then… a tunnel.
FRODO: He's led us this far, Sam.
SAM: Mr. Frodo, no.
FRODO: He's been true to his word.
FRODO: Lead the way, Sméagol.
It's easy to say that Frodo is just a great guy who wants to be nice to everyone so they can all get a long and be happy… but let's be real, with the way Frodo's been treating Sam recently, we don't think this is the case. Instead, Frodo might see Sméagol as a projection of his own future self. As Frodo becomes more and more engrossed in the grasp of the Ring, he begins to understand Gollum's plight and realizes that, in some twisted future, he will be the one riddled with hate for a new bearer and lust for his lost Precious.
Okay, so the Ring is a burden and carrying it must be awful—that band of gold seems to be literally draining the life out of Frodo. But Frodo himself is acting like a bit of a burden these days.
Not only has Frodo been distrusting of Sam (like during their argument over whether to follow Gollum on his little detour to Mordor), he's be outright violent. On the one hand, you can just say that it's just the curse of the Ring; Frodo is doing the best he can. He's carrying what is probably the darkest, most evil, dangerous object in all of Middle-earth. We get pretty cranky when we miss breakfast, so excusing Frodo for being a bit upset or forlorn seems only fair.
But on the other hand, can we really separate how the Ring affects Frodo from Frodo himself? Is the Ring possessing Frodo, taking control of his volition? It doesn't really seem like it. Maybe the Ring is just exacerbating his worst qualities. Or maybe this is some primal form of Frodo that's uncaged by the calling of the Ring.